There is only one way light can really slow down. And that is by interacting with the EM field of matter. I have been told it is absorption and emission of light in a medium that slows it down. But this doesn't actually CHANGE the SPEED of light. The other argument is that light travels a longer path as through a prism. This too is not the light really moving slower but just taking a different path.
It is true that light speed is slowed down by matter. It can even be halted. But I say that it is the EM field around matter that accomplishes this. It is a field effect.
Anybody want to add their two cents about this effect? Someone that knows about how science is now slowing light. What it is about the EM field that slows(resists) light? How are they interacting to accomplish this?
Regards.
Ive a theory that the 'aether' is more concentrated in transparent medium such as glass or water that causes light to slow down, then speed up upon departure from that object.
It is a field effect to an extent because as the 'aether' or hypothetical medium is concentrated in matter that is transparent to light and other electromagnetic frequencies. The 'aether' then gives rise to creating a field within that object's molecules that actually slows down light. Nothing to do with the absolute constant of 'c' or the velocity of EM in relativity theory because that constant has been found to shift in various physics experiment.
It is a field effect to an extent because as the 'aether' or hypothetical medium is concentrated in matter that is transparent to light and other electromagnetic frequencies. The 'aether' then gives rise to creating a field within that object's molecules that actually slows down light. Nothing to do with the absolute constant of 'c' or the velocity of EM in relativity theory because that constant has been found to shift in various physics experiment.
That was me above - I forgot to sign in.
Looking back at your question about EM fields slowing down light, well light is made up of transverse or sine wave magnetic and electric fields that are 90 degrees out of phase to each other. Therefore if an EM field is very intense as it forms around the source, then light would be affected to an extent. The effect is not really observable because light is a stable waveform that is not deflected easily by external EM fields.
Looking back at your question about EM fields slowing down light, well light is made up of transverse or sine wave magnetic and electric fields that are 90 degrees out of phase to each other. Therefore if an EM field is very intense as it forms around the source, then light would be affected to an extent. The effect is not really observable because light is a stable waveform that is not deflected easily by external EM fields.
Describing light encountering a transparent and non-transparent material is complicated as you need to look at what is happening at the particle level. I will try to explain it in a “light” version rather than the “heavy”, which requires a lengthy physical outline.
Before explaining that, a quick look at vibrations. In general we can say that everything vibrates in one way or another, down to the smallest particle. We may not sense all these vibrations as they are out of a frequency range or amplitude which we cannot sense.
Light is observed as a wave (EM), carried by the photon, but you and other masses will experience the wave as vibrations because they are coming towards you standing still in comparison with the light-speed, just as if you were holding the end of a rope and someone were shaking the rope at the other end, you feel the vibration.
Phonon is the mode of vibration in the lattice of atoms in a solid, i.e. a “resonance” frequency of the combinations of atoms and how they are arranged within the solid. There are many phonons in a solid. You can have same atoms in a solid but different arrangement, like hydrocarbons in a diamond or in graphite, one is transparent and the other not, the phonon is not the same in the two solids.
A transparent solid is a solid which has no phonons (modes of vibration) corresponding to the wave (or frequency) of light. If light hits a solid which has the phonons of light, the energy will be absorbed and amplifies the existing phonos (ultimately results as heat). If light hits a solid which has no corresponding phonos, the energy can not be absorbed permanently, and is therefore just temporarily absorbed only to be re-transmitted in the direction it came in (looking away from refraction). This temporary absorbtion and retransmission is slowing down the photon, and the denser the solid, the slower is the light passing through.
If the phonon (frequency) is very close to the lights frequency, some of the light’s energy will be absorbed and the balance of the energy is re-transmitted but with the frequency of the phonon, which may change the colour (frequency) of the light and reduce intensity.
At the surface of the transparent solid there is a slight disturbance in the phonon compared to inside the solid, which makes some of the the light reflect (bounced back to the media it came from)
Before explaining that, a quick look at vibrations. In general we can say that everything vibrates in one way or another, down to the smallest particle. We may not sense all these vibrations as they are out of a frequency range or amplitude which we cannot sense.
Light is observed as a wave (EM), carried by the photon, but you and other masses will experience the wave as vibrations because they are coming towards you standing still in comparison with the light-speed, just as if you were holding the end of a rope and someone were shaking the rope at the other end, you feel the vibration.
Phonon is the mode of vibration in the lattice of atoms in a solid, i.e. a “resonance” frequency of the combinations of atoms and how they are arranged within the solid. There are many phonons in a solid. You can have same atoms in a solid but different arrangement, like hydrocarbons in a diamond or in graphite, one is transparent and the other not, the phonon is not the same in the two solids.
A transparent solid is a solid which has no phonons (modes of vibration) corresponding to the wave (or frequency) of light. If light hits a solid which has the phonons of light, the energy will be absorbed and amplifies the existing phonos (ultimately results as heat). If light hits a solid which has no corresponding phonos, the energy can not be absorbed permanently, and is therefore just temporarily absorbed only to be re-transmitted in the direction it came in (looking away from refraction). This temporary absorbtion and retransmission is slowing down the photon, and the denser the solid, the slower is the light passing through.
If the phonon (frequency) is very close to the lights frequency, some of the light’s energy will be absorbed and the balance of the energy is re-transmitted but with the frequency of the phonon, which may change the colour (frequency) of the light and reduce intensity.
At the surface of the transparent solid there is a slight disturbance in the phonon compared to inside the solid, which makes some of the the light reflect (bounced back to the media it came from)
Hi - seems that some of us forgets to log in today - the above guest is Tor - sorry, forgot to log on
Hey tor, if that is how a medium slows down a photon how does a medium reflect a photon?
Tor,
Perhaps you should try the heavy version of your explanation; your light version is inconsistent and illogical. If you really want to explain these vibrations, you will have to do it yourself, not by using current texts. If Science had a Unified Theory of Vibrations, you could quote them and convey something useful.
Nick,
There is no way that light can "really slow down". Our concept of speed is inextricably combined with our concepts of time and distance. What you are talking about is the altering of the distance concept. Just put yourself in place of the "photon", and bounce yourself between two trampolines (no gravity) that are 1 meter apart. You will never travel more than 1 meter in 1 direction, so your speed is never more than 1 meter per 1 unit of time. Are you standing still? No, you are bouncing between 2 "reflective" mediums, at the rate of the original force. While overly simplified, this is all that they are doing with "slowing down" light.
TRoc
Perhaps you should try the heavy version of your explanation; your light version is inconsistent and illogical. If you really want to explain these vibrations, you will have to do it yourself, not by using current texts. If Science had a Unified Theory of Vibrations, you could quote them and convey something useful.
Nick,
There is no way that light can "really slow down". Our concept of speed is inextricably combined with our concepts of time and distance. What you are talking about is the altering of the distance concept. Just put yourself in place of the "photon", and bounce yourself between two trampolines (no gravity) that are 1 meter apart. You will never travel more than 1 meter in 1 direction, so your speed is never more than 1 meter per 1 unit of time. Are you standing still? No, you are bouncing between 2 "reflective" mediums, at the rate of the original force. While overly simplified, this is all that they are doing with "slowing down" light.
TRoc
Hi TRoc and Nick,
QUOTE (TRoc Posted on Nov 5 2005+ 03:13 PM)
Just put yourself in place of the "photon", and bounce yourself between two trampolines (no gravity) that are 1 meter apart. You will never travel more than 1 meter in 1 direction, so your speed is never more than 1 meter per 1 unit of time.
Ahem... according to which observer in which frame of reference?
Cheers
Ahem... according to which observer in which frame of reference?
Cheers
GE,
I am just using the analogy to mimic the eyeball of the scientist peering into the apparatus that contains the "slowed light". It will not matter from from what angle he/she views the experiment, it is the gross distortion of relative size between a human eye and a photon that produces the shamkommen (that is spelled wrong!; "illusion").
TRoc
I am just using the analogy to mimic the eyeball of the scientist peering into the apparatus that contains the "slowed light". It will not matter from from what angle he/she views the experiment, it is the gross distortion of relative size between a human eye and a photon that produces the shamkommen (that is spelled wrong!; "illusion").
TRoc
QUOTE (Nick+Nov 3 2005, 02:51 AM)
It is a field effect.
By the Aether wave theory the light speed decreases in the massive medium, because part of its energy spreads across the convoluted dimensions by the high local speed. Therefore the overall light speed decreases, whereas the gravity speed retains the same, as discussed previously.
By the Aether wave theory the light speed decreases in the massive medium, because part of its energy spreads across the convoluted dimensions by the high local speed. Therefore the overall light speed decreases, whereas the gravity speed retains the same, as discussed previously.
QUOTE (TRoc+Nov 5 2005, 03:13 PM)
Nick,
There is no way that light can "really slow down". Our concept of speed is inextricably combined with our concepts of time and distance. What you are talking about is the altering of the distance concept. Just put yourself in place of the "photon", and bounce yourself between two trampolines (no gravity) that are 1 meter apart. You will never travel more than 1 meter in 1 direction, so your speed is never more than 1 meter per 1 unit of time. Are you standing still? No, you are bouncing between 2 "reflective" mediums, at the rate of the original force. While overly simplified, this is all that they are doing with "slowing down" light.
TRoc
TRoc, you are describing gravity. My point is that EM force is affecting light in a completely different way than gravity. The relationship between light and gravity according to THE UNIFIED FIELD Einstein was looking for is that: LIGHT FALLS. That is what you are describing. But it is different when you consider the EM field affecting the speed of an EM wave. It is completely different. There is no time slowdown to be accounted for there.
And Zephy,
You are saying that the motion of the particles is very high "locally" causing them to catch up to the light?
Look at what Einstein said: Even if you move through space at a high speed light's speed remains a constant.
Therefore you don't know what you are talking about Zeph.
There is no way that light can "really slow down". Our concept of speed is inextricably combined with our concepts of time and distance. What you are talking about is the altering of the distance concept. Just put yourself in place of the "photon", and bounce yourself between two trampolines (no gravity) that are 1 meter apart. You will never travel more than 1 meter in 1 direction, so your speed is never more than 1 meter per 1 unit of time. Are you standing still? No, you are bouncing between 2 "reflective" mediums, at the rate of the original force. While overly simplified, this is all that they are doing with "slowing down" light.
TRoc
TRoc, you are describing gravity. My point is that EM force is affecting light in a completely different way than gravity. The relationship between light and gravity according to THE UNIFIED FIELD Einstein was looking for is that: LIGHT FALLS. That is what you are describing. But it is different when you consider the EM field affecting the speed of an EM wave. It is completely different. There is no time slowdown to be accounted for there.
And Zephy,
You are saying that the motion of the particles is very high "locally" causing them to catch up to the light?
Look at what Einstein said: Even if you move through space at a high speed light's speed remains a constant.
Therefore you don't know what you are talking about Zeph.
QUOTE (Nick+Nov 7 2005, 06:27 AM)
Even if you move through space at a high speed light's speed remains a constant.
Hi, Nick
You objection has nothing to do with light speed problem, as you're not moving, but light instead.
There are two equivalent models of describing the light speed and space-time relationship, but if we suppose, the light speed remains constant, everything is OK, just because the space-time in convoluted dimension space-time metric is compressed.
Hi, Nick
You objection has nothing to do with light speed problem, as you're not moving, but light instead.
There are two equivalent models of describing the light speed and space-time relationship, but if we suppose, the light speed remains constant, everything is OK, just because the space-time in convoluted dimension space-time metric is compressed.
Nick,
No.
TRoc
No.
TRoc
The original question asked "what is it about matter..." Matter is mass, mass assumes gravity, yet in your discussions you want to discount gravity and stick closely to EM effects on light. Gravity must play a role in any EM effect. Apparently you meant "what is it about EM that slows light" Even so, mass is involved, no matter how small and quantized, and where there is mass there is gravity. So you can't factor it out in discussions of light and EM. Gravity bends light, but does the light slow down? Apparently not. Is a kind of Bernoulli effect at work in creating a gravity lens? Apparently so. But what are these references to aether? Didn't E'stein prove there was no aether?
Let me shed some light (hehe) on the subject.
The speed of light is inversely proportional to the electric permittivity (the ease of propagating an electric field) and magnetic permeability (the ease of propagating a magnetic field). The equations for this are at http://hyperphysics.phy-astr.gsu.edu/hbase/hph.html.
All of the Electromagnetic Spectrum (EM) travels at the speed of light. A material is transparent to any EM frequency (F) if it is not absorbed or reflected by said material.
Dispersion (prism effects) tells us that the speed of light at any F for any transparent material is unique for that combination.
The frequency of light cannot be changed (excluding red-shift, black holes, and relativistic effects) so the wavelength of F is shortened to reflect the slower speed of light in the transparent material.
The "wavelength" is affected by the changes in permittivity and permeability. The speed of light is still the speed of light in any material that is transparent to F.
Now how matter increases permittivity and/or permeability and how they interact with light is open for discussion.
As a side note that same transparent material will reflect a horizontally polarized beam of light at the Brewster's angle for that F and material. This may shed some light
on the above discussion.
The speed of light is inversely proportional to the electric permittivity (the ease of propagating an electric field) and magnetic permeability (the ease of propagating a magnetic field). The equations for this are at http://hyperphysics.phy-astr.gsu.edu/hbase/hph.html.
All of the Electromagnetic Spectrum (EM) travels at the speed of light. A material is transparent to any EM frequency (F) if it is not absorbed or reflected by said material.
Dispersion (prism effects) tells us that the speed of light at any F for any transparent material is unique for that combination.
The frequency of light cannot be changed (excluding red-shift, black holes, and relativistic effects) so the wavelength of F is shortened to reflect the slower speed of light in the transparent material.
The "wavelength" is affected by the changes in permittivity and permeability. The speed of light is still the speed of light in any material that is transparent to F.
Now how matter increases permittivity and/or permeability and how they interact with light is open for discussion.
As a side note that same transparent material will reflect a horizontally polarized beam of light at the Brewster's angle for that F and material. This may shed some light
on the above discussion.
Electric permittivity (the ease of propagating an electric field) and magnetic permeability(ease of propagating a magnetic field)
That is the answer Montec. Thankyou!
Light interacts with the different permittivity and permeability of the EM field around charged matter.
I like to point out: Light is a DUAL WAVE. It is both a magnetic wave and an electric wave together.
That is the answer Montec. Thankyou!
Light interacts with the different permittivity and permeability of the EM field around charged matter.
I like to point out: Light is a DUAL WAVE. It is both a magnetic wave and an electric wave together.
Above answer is correct. But I'll put it simple:
Energy is vibration.
Light is high-frequency energy.
Matter is condensed light.
Condensed light is low frequency energy.
If light hits matter a small part of the collision will result in dissonance, reducing the pure/real light's speed. Most part of it will pass through (resonance).
In the end is vibration colliding with vibration.
Energy is vibration.
Light is high-frequency energy.
Matter is condensed light.
Condensed light is low frequency energy.
If light hits matter a small part of the collision will result in dissonance, reducing the pure/real light's speed. Most part of it will pass through (resonance).
In the end is vibration colliding with vibration.
QUOTE (gushens+Nov 8 2005, 06:37 AM)
Above answer is correct. But I'll put it simple:
Energy is vibration.
Light is high-frequency energy.
Matter is condensed light.
Condensed light is low frequency energy.
If light hits matter a small part of the collision will result in dissonance, reducing the pure/real light's speed. Most part of it will pass through (resonance).
In the end is vibration colliding with vibration.
You're having some problems gushens:
#1:Light is high frequency energy?
Light doesn't have to be high frequency
@2:Condenced light is low frequency energy?
Condensed light isn't low frequency energy
it is actually mass. By Einstein's definition
mass is a form of concentrated energy.
You mixed it up!
Maybe you know more? How does the vibration frequency of matter affect the vibration frequency of light and vice versa when one is a probability wave?
Energy is vibration.
Light is high-frequency energy.
Matter is condensed light.
Condensed light is low frequency energy.
If light hits matter a small part of the collision will result in dissonance, reducing the pure/real light's speed. Most part of it will pass through (resonance).
In the end is vibration colliding with vibration.
You're having some problems gushens:
#1:Light is high frequency energy?
Light doesn't have to be high frequency
@2:Condenced light is low frequency energy?
Condensed light isn't low frequency energy
it is actually mass. By Einstein's definition
mass is a form of concentrated energy.
You mixed it up!
Maybe you know more? How does the vibration frequency of matter affect the vibration frequency of light and vice versa when one is a probability wave?
dont forget to mention that light is also a particle, not just a wave.
I too have been looking for the answer to this question for the last five years.
As already suggested
"Light interacts with the different permittivity and permeability of the EM field around charged matter"
- I have rejected this theory as 'too simple' because it seems logical to suppose that thermal vibrations in any transmission medium would be superimposed on the beam of light in the form of random (small) changes in frequency. In any appreciable distance the light will have undergone billions of interactions and the cumulative effect would be (by my reckoning) that a beam of light would be rapidly dispersed both in direction and frequency.
While reading constants for permittivity and permeability may enable us to predict WHAT will happen I don't think it explains WHY it happens.
As already suggested
"Light interacts with the different permittivity and permeability of the EM field around charged matter"
- I have rejected this theory as 'too simple' because it seems logical to suppose that thermal vibrations in any transmission medium would be superimposed on the beam of light in the form of random (small) changes in frequency. In any appreciable distance the light will have undergone billions of interactions and the cumulative effect would be (by my reckoning) that a beam of light would be rapidly dispersed both in direction and frequency.
While reading constants for permittivity and permeability may enable us to predict WHAT will happen I don't think it explains WHY it happens.
QUOTE (Guest_paul+Nov 8 2005, 11:12 AM)
dont forget to mention that light is also a particle, not just a wave.
As Einstein said: Light is a wave packet. So we can have large photons; the size of the wave.
As Einstein said: Light is a wave packet. So we can have large photons; the size of the wave.
Hello Nick,
From my point of view, as long as we are agreed that a photon is either detected or not detected then we are at least travelling in the same direction. I suggest that the wavelength model is helpful in describing the time when we are most likely to detect a photon but has no physical significance in terms of length, breadth or height of the actual photon. The photon remains essentially a probability of an effect. If we suspected a meteorite would pass within a million miles of earth this would not make the meteorite a million miles wide. A photon is a yes or no event, it cannot be a miilion miles wide or long.
Everything I say may well be wrong, if this turns out to be the case then please forgive me!
-c2.
From my point of view, as long as we are agreed that a photon is either detected or not detected then we are at least travelling in the same direction. I suggest that the wavelength model is helpful in describing the time when we are most likely to detect a photon but has no physical significance in terms of length, breadth or height of the actual photon. The photon remains essentially a probability of an effect. If we suspected a meteorite would pass within a million miles of earth this would not make the meteorite a million miles wide. A photon is a yes or no event, it cannot be a miilion miles wide or long.
Everything I say may well be wrong, if this turns out to be the case then please forgive me!
-c2.
My problem is that I can't separate anything.
Light must be high frequency for us! It's associated with maximum peaks of energy. Heat, velocity. How couldn't it be?
Of course that low and high are relative to the observer.
Light must be high frequency for us! It's associated with maximum peaks of energy. Heat, velocity. How couldn't it be?
Of course that low and high are relative to the observer.
@2:Condenced light is low frequency energy?
Condensed light isn't low frequency energy
it is actually mass. By Einstein's definition
mass is a form of concentrated energy.
If mass is concentrated energy than it must be low frequency energy to be able to appear to us as distinct corpuscular objects, because low frequency is related to dual perception of reality. Low frequency isn't only associated to bigger wavelengths, it's also associated with bigger amplitudes. Waves with bigger amplitude are going (asymptotically) all the way up (infinite positive y value for peak) and down (infinite negative y value for valley) so we have two infinite journeys, one up and one down. These journeys are dual perception of reality because you go up all the way up and then you go all the way down, and the alternation is not simultaneous. If you have a non-simultaneous alternation you are dealing with a pendulum that is still moving between two extremes and this is dual perception! Dual perception of reality is associated with contrasting objects and backgrounds and we assume right away that these objects have more mass than "empty" space sorrounding them.
This is how I see mass as low frequency energy.
So if condensed light is mass than condensed light is low frequency energy. Then also light that is not condensed must be high frequency energy!
Of course that low and high are relative to the observer.
I'll have to read what is a probability wave. I don't know that.
QUOTE
#1:Light is high frequency energy?
Light doesn't have to be high frequency
Light doesn't have to be high frequency
Light must be high frequency for us! It's associated with maximum peaks of energy. Heat, velocity. How couldn't it be?
Of course that low and high are relative to the observer.
QUOTE (->
| QUOTE |
| #1:Light is high frequency energy? Light doesn't have to be high frequency |
Light must be high frequency for us! It's associated with maximum peaks of energy. Heat, velocity. How couldn't it be?
Of course that low and high are relative to the observer.
@2:Condenced light is low frequency energy?
Condensed light isn't low frequency energy
it is actually mass. By Einstein's definition
mass is a form of concentrated energy.
If mass is concentrated energy than it must be low frequency energy to be able to appear to us as distinct corpuscular objects, because low frequency is related to dual perception of reality. Low frequency isn't only associated to bigger wavelengths, it's also associated with bigger amplitudes. Waves with bigger amplitude are going (asymptotically) all the way up (infinite positive y value for peak) and down (infinite negative y value for valley) so we have two infinite journeys, one up and one down. These journeys are dual perception of reality because you go up all the way up and then you go all the way down, and the alternation is not simultaneous. If you have a non-simultaneous alternation you are dealing with a pendulum that is still moving between two extremes and this is dual perception! Dual perception of reality is associated with contrasting objects and backgrounds and we assume right away that these objects have more mass than "empty" space sorrounding them.
This is how I see mass as low frequency energy.
So if condensed light is mass than condensed light is low frequency energy. Then also light that is not condensed must be high frequency energy!
Of course that low and high are relative to the observer.
I'll have to read what is a probability wave. I don't know that.
Hello Confused2
Permittivity and permeability are fields made up from millions of point sources (atoms, etc.) that each generate overlapping fields. Normally vibrating atoms will not disrupt these fields. However if the vibration becomes large enough then the overall field will become disrupted and the EM wave will be subjected to a large number of refractions.
Permittivity and permeability are fields made up from millions of point sources (atoms, etc.) that each generate overlapping fields. Normally vibrating atoms will not disrupt these fields. However if the vibration becomes large enough then the overall field will become disrupted and the EM wave will be subjected to a large number of refractions.
I should point out that the permittivity and permeability fields in my last post are just the changes made by matter to the permittivity and permeability of a vacuum.
Hello Gushens,
Re Probability waves
Most of what I think I know (or pretend to know) about probability waves comes from a book called QED by Richard Feynman - it is fairly old now, there may be better books.
Hello Montec,
Thank you.
As you suggested earlier..
"how matter increases permittivity and/or permeability and how they interact with light is open for discussion" - I'd like that discussion very much.
May I offer a 10cm cube of quartz glass (we may wish to heat it later), optically perfect on every face. 1 cm in from one corner is a spherical lump of carbon (black) which is 1mm in diameter. We fire single photons through the centre of one of the faces and measure 'everything' as the photon emerges (or does not emerge) at the opposite face.
The general view seems to be that there are 'temporary trappy states' in the glass which the photon will sniff at and average out to determine what velocity it will choose to travel at - comments invited. If we increase the number of photons (eg a laser pulse) will all the 'trappy states' become occupied and the untrapped photons travel unimpeded - again - comments invited.
-c2
Re Probability waves
Most of what I think I know (or pretend to know) about probability waves comes from a book called QED by Richard Feynman - it is fairly old now, there may be better books.
Hello Montec,
Thank you.
As you suggested earlier..
"how matter increases permittivity and/or permeability and how they interact with light is open for discussion" - I'd like that discussion very much.
May I offer a 10cm cube of quartz glass (we may wish to heat it later), optically perfect on every face. 1 cm in from one corner is a spherical lump of carbon (black) which is 1mm in diameter. We fire single photons through the centre of one of the faces and measure 'everything' as the photon emerges (or does not emerge) at the opposite face.
The general view seems to be that there are 'temporary trappy states' in the glass which the photon will sniff at and average out to determine what velocity it will choose to travel at - comments invited. If we increase the number of photons (eg a laser pulse) will all the 'trappy states' become occupied and the untrapped photons travel unimpeded - again - comments invited.
-c2
Whatever theory that tries to explain light and matter interaction must include :
Brillouin scattering
Rayleigh Scattering
Mie Scattering
Ramon Scattering
Compton Scattering
A clue to theory is that Rayleigh Scattering affects higher frequencies more than lower frequencies just like Dispersion in optics.
Brillouin scattering
Rayleigh Scattering
Mie Scattering
Ramon Scattering
Compton Scattering
A clue to theory is that Rayleigh Scattering affects higher frequencies more than lower frequencies just like Dispersion in optics.
Montec:
Regarding your post of Nov 9 2005, 06:28 PM
Do you mean Raman Scattering rather than Ramon Scattering?
Regarding your post of Nov 9 2005, 06:28 PM
Do you mean Raman Scattering rather than Ramon Scattering?
The block of glass has no particles of the right sort of size for Mie, Rayleigh or Raman scattering to occur so we are saved from them. Our light is of low enough energy that we're not knocking out electrons so we don't have any Compton effect and we can keep our glass at a steady state so we don't have to worry about Brillouin scattering unless we really want to.
I certainly take your point that these all involve light-matter interaction and they all undeniably result in scattering. We could certainly creep up on interactions that do not cause scattering by looking at all the effects that do but I fear we might find ourselves no further forward at the end of the process.
In the light of the scattering effects already noted it seems likely that the creature we (or at least I) seek must be either substantially smaller then the wavelength of light or substantially bigger. My instinct is go for substantially bigger because..
It seems the reflection from the surface of a sheet of glass varies with the thickness of the glass, reflectance (?) changing by a few percent as the thickness is increased by half a wavelength or so. I think Feynman (in QED) claims this effect has been tested up to several feet and (I think) still exists when single photons are fired at the glass. Other experimental evidence most welcome here. If the effect is as I describe it then photons clearly know more about the piece of glass than I feel they have any right to. The next question would then be
"What could they possibly know that makes 'em slow down?"
Clearly the answer to the 'big effect question' could be that they know about the small effect..
I think I've gone on long enough.. still with me?
(Guest wasn't me, incidentally. Got enough of my own typos to worry about)
Edit.. I'm going to be really miffed if it turns out they know about epsilon and mu and that's the end of the story.
I certainly take your point that these all involve light-matter interaction and they all undeniably result in scattering. We could certainly creep up on interactions that do not cause scattering by looking at all the effects that do but I fear we might find ourselves no further forward at the end of the process.
In the light of the scattering effects already noted it seems likely that the creature we (or at least I) seek must be either substantially smaller then the wavelength of light or substantially bigger. My instinct is go for substantially bigger because..
It seems the reflection from the surface of a sheet of glass varies with the thickness of the glass, reflectance (?) changing by a few percent as the thickness is increased by half a wavelength or so. I think Feynman (in QED) claims this effect has been tested up to several feet and (I think) still exists when single photons are fired at the glass. Other experimental evidence most welcome here. If the effect is as I describe it then photons clearly know more about the piece of glass than I feel they have any right to. The next question would then be
"What could they possibly know that makes 'em slow down?"
Clearly the answer to the 'big effect question' could be that they know about the small effect..
I think I've gone on long enough.. still with me?
(Guest wasn't me, incidentally. Got enough of my own typos to worry about)
Edit.. I'm going to be really miffed if it turns out they know about epsilon and mu and that's the end of the story.
Guest:
Excuse me it is Raman Scattering. (Dang spellchecker)
Confused2:
What I was getting at is that whatever effect we see with spread out atoms (gases) must also effect transparent solids. IE for a single molecule we have Rayleigh scattering and for a transparent solid we have dispersion. Both of which act the same way on light.
If I remember right from my laser classes they can make mirrors for a particular frequency by using a series of half wave coatings with different indexes of refraction. Does this pertain to your question?
Excuse me it is Raman Scattering. (Dang spellchecker)
Confused2:
What I was getting at is that whatever effect we see with spread out atoms (gases) must also effect transparent solids. IE for a single molecule we have Rayleigh scattering and for a transparent solid we have dispersion. Both of which act the same way on light.
If I remember right from my laser classes they can make mirrors for a particular frequency by using a series of half wave coatings with different indexes of refraction. Does this pertain to your question?
Yo!
Oh dear! Did I start this frequency dependence problem, if so then I'm sorry, I didn't mean to. Can I swiftly substitute a trickle of photons from a laser source for whatever we might have imagined to be there previously? My point was that the named effects you drew attention to do not SEEM to occur in the sample of glass which I hoped we might examine closely. I hadn't really considered what we might see if we replaced the atmosphere with a 25 mile thick layer of glass.
My humble experiment involved a 10cm block of glass. Under the circumstances I think I am correct in saying that a beam of monochromatic light will pass through the glass and appear at the other side pretty much as it entered but demonstrably passing through the glass at a rate less than that which we would expect in free space.
My interpretation of the difference between Rayleigh scattering and dispersion is that in the case of Rayleigh scattering the effect will persist even if the light source is monochromatic whereas dispersion will not. Rayleigh scattering results in energy being diverted away from the original direction of the beam whereas dispersion does not. Please correct me if I seem to have misunderstood either effect. Without help I'm unable to use either effect to explain the slowing down of light in glass. I appreciate that we are looking for clues that might provide the answer - over to you on this one.
The only Internet reference I can find to the thickness dependant reflectivity of glass effect to which I referred is at
http://www.petergh.f2s.com/glasstext.htm.
As the effect is either controversial (possibly fictitious) or generally unknown then consider it discarded.
The half wave coatings give a clue about the ability of a photon (forgive me) to interfere with itself but this is rather less dramatic than I had hoped for.
At this stage my weapons are blunt and plan B has not yet emerged. I hope someone else can do better.
- c2
Oh dear! Did I start this frequency dependence problem, if so then I'm sorry, I didn't mean to. Can I swiftly substitute a trickle of photons from a laser source for whatever we might have imagined to be there previously? My point was that the named effects you drew attention to do not SEEM to occur in the sample of glass which I hoped we might examine closely. I hadn't really considered what we might see if we replaced the atmosphere with a 25 mile thick layer of glass.
My humble experiment involved a 10cm block of glass. Under the circumstances I think I am correct in saying that a beam of monochromatic light will pass through the glass and appear at the other side pretty much as it entered but demonstrably passing through the glass at a rate less than that which we would expect in free space.
My interpretation of the difference between Rayleigh scattering and dispersion is that in the case of Rayleigh scattering the effect will persist even if the light source is monochromatic whereas dispersion will not. Rayleigh scattering results in energy being diverted away from the original direction of the beam whereas dispersion does not. Please correct me if I seem to have misunderstood either effect. Without help I'm unable to use either effect to explain the slowing down of light in glass. I appreciate that we are looking for clues that might provide the answer - over to you on this one.
The only Internet reference I can find to the thickness dependant reflectivity of glass effect to which I referred is at
http://www.petergh.f2s.com/glasstext.htm.
As the effect is either controversial (possibly fictitious) or generally unknown then consider it discarded.
The half wave coatings give a clue about the ability of a photon (forgive me) to interfere with itself but this is rather less dramatic than I had hoped for.
At this stage my weapons are blunt and plan B has not yet emerged. I hope someone else can do better.
- c2
Hehe, here is a link to the info I found for Rayleigh scattering.
http://hyperphysics.phy-astr.gsu.edu/hbase.../blusky.html#c2
I looked at the site you gave and the increasing and decreasing percentages of reflected photons looks like a harmonic wave function of some sort between the laser beam and the glass.
I liked his explanation of glass being similar to the atmosphere.
http://hyperphysics.phy-astr.gsu.edu/hbase.../blusky.html#c2
I looked at the site you gave and the increasing and decreasing percentages of reflected photons looks like a harmonic wave function of some sort between the laser beam and the glass.
I liked his explanation of glass being similar to the atmosphere.
I would love to try to build a theory based on the 'thick glass' experiment but I'd like to know where the experiment was done, by whom, who repeated it and whether or not they got the same result. There is enough rubbish on the internet to support almost any theory - I don't really want to get too involved with the results of experiments which may have been performed by visiting aliens or (possibly) imagined by Feynman to make his lectures more interesting.
I very much like the way you come back to Rayleigh scattering. As someone else on this site suggested - if you can think it it must mean something - but what?
If you can suggest how the Rayleigh off-axis radiation might cancel out I'd be right there with you. One thought is that with a large number of molecules the sum of the probability of scattering in any particular direction might be reduced to (virtually) zero - the move from gas to solid would prove to be an advantage if this is the case. Not rejected - needs some thinking about - suggestions welcome.
-c2.
I very much like the way you come back to Rayleigh scattering. As someone else on this site suggested - if you can think it it must mean something - but what?
If you can suggest how the Rayleigh off-axis radiation might cancel out I'd be right there with you. One thought is that with a large number of molecules the sum of the probability of scattering in any particular direction might be reduced to (virtually) zero - the move from gas to solid would prove to be an advantage if this is the case. Not rejected - needs some thinking about - suggestions welcome.
-c2.
Hello “qwerty the warrior babe”,
Been travelling – but see you have some input from different posters with different ideas in the meantime. I’ll get back to your original question as it looks like the thread got off its theme.
We know a lot about light and its interactions with solids, otherwise we would not have been able to design , among many things those advanced CMOS and CCD sensors for photo applications, filters, optics, etc. etc.
Disregarding Troc’s unqualified comments I continue the path of “light-weight” versions of the explanations for light encountering solids.
Reflection of light is happening in various ways depending on the solid. It could be a dielectric solid or a metal, and part of the reflection is dependent on the surface of the solid.
If you look at a solid’s reflection you will find two different, one being the specular reflection, i.e. bouncing the light off the surface containing all colours of the source (the “glare”).
The other “reflection” is the colour of the solid which you will see from all angles. When a composite light (all colours) hits a dielectric solid, most of the frequencies (colours) will be absorbed by the solid and converted to heat. The wavelength that coincides with the phonon (lattice vibration of the atom structure) at the surface, will be temporarily absorbed and retransmitted in a diffuse manner (in all directions) in the wavelength (colour) of the phonon.
Reflections from metals are slightly different from dielectric solids, as metals have conduction electrons, which re-radiates the identical light but with a 180 degree phase shift.
TRoc,
If my explanation is inconsistent and illogical you must explain why.
If you do not know phonon in solids, you should look it up, its common physics.
Your “bouncing” explanation is not correct, the energy is absorbed temporarily and re-emitted, that’s why you have the delay, hence light is travelling slower through the solid. In an experiment light was slowed down to 120 000 km/s in a normal diamond, and in an experiments by a research team at the Rowland Institute for Science and Harvard University light waves were brought down to a 2 km/h crawl by putting them through a specially prepared haze of ultracold sodium atoms
Been travelling – but see you have some input from different posters with different ideas in the meantime. I’ll get back to your original question as it looks like the thread got off its theme.
We know a lot about light and its interactions with solids, otherwise we would not have been able to design , among many things those advanced CMOS and CCD sensors for photo applications, filters, optics, etc. etc.
Disregarding Troc’s unqualified comments I continue the path of “light-weight” versions of the explanations for light encountering solids.
Reflection of light is happening in various ways depending on the solid. It could be a dielectric solid or a metal, and part of the reflection is dependent on the surface of the solid.
If you look at a solid’s reflection you will find two different, one being the specular reflection, i.e. bouncing the light off the surface containing all colours of the source (the “glare”).
The other “reflection” is the colour of the solid which you will see from all angles. When a composite light (all colours) hits a dielectric solid, most of the frequencies (colours) will be absorbed by the solid and converted to heat. The wavelength that coincides with the phonon (lattice vibration of the atom structure) at the surface, will be temporarily absorbed and retransmitted in a diffuse manner (in all directions) in the wavelength (colour) of the phonon.
Reflections from metals are slightly different from dielectric solids, as metals have conduction electrons, which re-radiates the identical light but with a 180 degree phase shift.
TRoc,
If my explanation is inconsistent and illogical you must explain why.
If you do not know phonon in solids, you should look it up, its common physics.
Your “bouncing” explanation is not correct, the energy is absorbed temporarily and re-emitted, that’s why you have the delay, hence light is travelling slower through the solid. In an experiment light was slowed down to 120 000 km/s in a normal diamond, and in an experiments by a research team at the Rowland Institute for Science and Harvard University light waves were brought down to a 2 km/h crawl by putting them through a specially prepared haze of ultracold sodium atoms
Tor,
First off, your last post was very concise.
I apologize for not taking the time to "qualify" my disagreements. It was not personal; it was more a statement against the current, too numerous, models. If we wish to simplify, and unify the theories is Science, the first step is to reduce the terminologies for the same phenomenon. You started off well.. using the term "vibrations" as a general term. I just suggest sticking to that. I will try a fast approach to where, specifically, I saw some "misleading" (to the student) labels.
Quote "Light is observed as a wave (EM), carried by the photon, but you and other masses will experience the wave as vibrations because they are coming towards you standing still in comparison with the light-speed, just as if you were holding the end of a rope and someone were shaking the rope at the other end, you feel the vibration."
Light IS.. AS wave.. BY photon In this context, it seems you are talking about 3 different things. Do you mean, by "light", frequencies in the "visible" range? Is the "photon" a quantum of energy, or a "vehicle" that vibrations travel inside?
"Phonon is the mode of vibration in the lattice of atoms in a solid, i.e. a “resonance” frequency of the combinations of atoms and how they are arranged within the solid. There are many phonons in a solid. You can have same atoms in a solid but different arrangement, like hydrocarbons in a diamond or in graphite, one is transparent and the other not, the phonon is not the same in the two solids."
Phonon=Resonance frequency=Normal mode, followed by "there are many phonons". Doesn't that imply more than one resonant frequency? I agree that the geometry of the lattice will affect the resonance, because the outermost atoms are in different form than the inner. You also have the boundary condition to contend with. If phonons are the vibrations within a crystal lattice, then, certainly, you can have more than 1. If phonons are "the resonant frequency" (normal mode), in a solid, then you have just 1.
"A transparent solid is a solid which has no phonons (modes of vibration) corresponding to the wave (or frequency) of light. If light hits a solid which has the phonons of light, the energy will be absorbed and amplifies the existing phonos (ultimately results as heat). If light hits a solid which has no corresponding phonos, the energy can not be absorbed permanently, and is therefore just temporarily absorbed only to be re-transmitted in the direction it came in (looking away from refraction). This temporary absorption and retransmission is slowing down the photon, and the denser the solid, the slower is the light passing through."
Are you saying that a transparent solid absorbs NOTHING? The "fundamental" atoms are somehow different? What about the "ordered" structure, doesn't that play an important role in "transparency"? Bringing in the terms "permanent" and "temporary" doesn't help matters. Zero absorption = 100% reflection. Heat is just more vibration.
Mainly, I think you know what you are talking about, but need to refine your presentation for the "student" level questioner.
TRoc
First off, your last post was very concise.
I apologize for not taking the time to "qualify" my disagreements. It was not personal; it was more a statement against the current, too numerous, models. If we wish to simplify, and unify the theories is Science, the first step is to reduce the terminologies for the same phenomenon. You started off well.. using the term "vibrations" as a general term. I just suggest sticking to that. I will try a fast approach to where, specifically, I saw some "misleading" (to the student) labels.
Quote "Light is observed as a wave (EM), carried by the photon, but you and other masses will experience the wave as vibrations because they are coming towards you standing still in comparison with the light-speed, just as if you were holding the end of a rope and someone were shaking the rope at the other end, you feel the vibration."
Light IS.. AS wave.. BY photon In this context, it seems you are talking about 3 different things. Do you mean, by "light", frequencies in the "visible" range? Is the "photon" a quantum of energy, or a "vehicle" that vibrations travel inside?
"Phonon is the mode of vibration in the lattice of atoms in a solid, i.e. a “resonance” frequency of the combinations of atoms and how they are arranged within the solid. There are many phonons in a solid. You can have same atoms in a solid but different arrangement, like hydrocarbons in a diamond or in graphite, one is transparent and the other not, the phonon is not the same in the two solids."
Phonon=Resonance frequency=Normal mode, followed by "there are many phonons". Doesn't that imply more than one resonant frequency? I agree that the geometry of the lattice will affect the resonance, because the outermost atoms are in different form than the inner. You also have the boundary condition to contend with. If phonons are the vibrations within a crystal lattice, then, certainly, you can have more than 1. If phonons are "the resonant frequency" (normal mode), in a solid, then you have just 1.
"A transparent solid is a solid which has no phonons (modes of vibration) corresponding to the wave (or frequency) of light. If light hits a solid which has the phonons of light, the energy will be absorbed and amplifies the existing phonos (ultimately results as heat). If light hits a solid which has no corresponding phonos, the energy can not be absorbed permanently, and is therefore just temporarily absorbed only to be re-transmitted in the direction it came in (looking away from refraction). This temporary absorption and retransmission is slowing down the photon, and the denser the solid, the slower is the light passing through."
Are you saying that a transparent solid absorbs NOTHING? The "fundamental" atoms are somehow different? What about the "ordered" structure, doesn't that play an important role in "transparency"? Bringing in the terms "permanent" and "temporary" doesn't help matters. Zero absorption = 100% reflection. Heat is just more vibration.
Mainly, I think you know what you are talking about, but need to refine your presentation for the "student" level questioner.
TRoc
Hello Confused2
What I'm angling for is a relationship between an atom's diameter of effect associated with the permittivity and permeability change and density (spacing between atoms). I would also like to know whether different elements have different effect radii. Trying to zero in on the basic mechanism that causes the effect.
Edit:
What I'm assuming is that the Rayleigh scattering and dispersion are caused by the same underlying force.
What I'm angling for is a relationship between an atom's diameter of effect associated with the permittivity and permeability change and density (spacing between atoms). I would also like to know whether different elements have different effect radii. Trying to zero in on the basic mechanism that causes the effect.
Edit:
What I'm assuming is that the Rayleigh scattering and dispersion are caused by the same underlying force.
TRoc and others
My understanding of the phonon theory so far is..
Photon arrives from the east
photon absorbed by unspecified process
phonon created
time passes
phonon wakes up
phonon sends photon off in a westerly direction
phonon dies
Am I close?
When I was at school we had a thing called something like "the theorum of equipartition" which I think suggested that energy would be evenly distributed through all degrees of freedom One problem with these phonons is that you can see quite clearly through a tube of silica glass even when it is at 1200C (white hot) - why aren't these phonons radiating then?
Montec - yes, some crystals do have different refractive indexes in different directions.
-c2
My understanding of the phonon theory so far is..
Photon arrives from the east
photon absorbed by unspecified process
phonon created
time passes
phonon wakes up
phonon sends photon off in a westerly direction
phonon dies
Am I close?
When I was at school we had a thing called something like "the theorum of equipartition" which I think suggested that energy would be evenly distributed through all degrees of freedom One problem with these phonons is that you can see quite clearly through a tube of silica glass even when it is at 1200C (white hot) - why aren't these phonons radiating then?
Montec - yes, some crystals do have different refractive indexes in different directions.
-c2
After a bit of thought I'll have a go at answering my own question.
I suggest..
The glass does not radiate at 1200C because it can't - it is pretty much the opposite of a black body. If the glass can't radiate it is reasonable to suppose that it can't absorb either. Any proposed mechanism of photon absorption/emission needs some careful explanation because it seems probable (to me) that the glass itself is physically incapable of supporting this process.
-c2
I suggest..
The glass does not radiate at 1200C because it can't - it is pretty much the opposite of a black body. If the glass can't radiate it is reasonable to suppose that it can't absorb either. Any proposed mechanism of photon absorption/emission needs some careful explanation because it seems probable (to me) that the glass itself is physically incapable of supporting this process.
-c2
Hi Confused2,
First, regarding your understanding of the ”phonon theory” (which in fact is not a theory), you are not very close.
Let me first explain phonons again. The atoms in a solid are not locked in a rigid pattern but can oscillate around their average position in the solid. Most solids have periodic arrays of atoms which forms a crystal lattice. Amorphous solids and glasses are exceptions. When a group of atoms oscillates in a synchronized way, it’s called lattice vibration. Different groups of atoms can oscillate (vibrate) with different frequencies which we call phonons. These vibrations are important for many properties of the solid, among them the transport of heat.
Now, to a “superlight” explanation of transparency:
Imagine the group of atoms as groups of people, each group dancing to their own rhythm in a room. Into the room comes a photon dancing in the rhythm of light (frequency of light). It tries to join a group in the dance (delayed), but they have a different rhythm, is rejected and sent to the next group in the same direction, and the same is repeated until the phonon escapes the room on the other side. If a group had the same rhythm as the phonon, the phonon would have joined the group in the dance and produced heat.
Since the lattice of atom arrangements in amorphous solids and glass is different from other solids, they do not have frequencies corresponding to the frequency of light; hence it is rejected after “test” and passed through.
When you mention that the phonons radiate, this is wrong, the amplitudes of the vibration is nowhere near a possibility to radiate outside the lattice.
As you mentioned, pure silica (SiO2) also called fused quarts will be transparent at 1200C (melting temperature is 2000C) while normal window glass melts at approx 1000C. Quarts glass will also be transparent to ultra violet light (no phonons corresponding to ultra violet light (200-380nm wavelength), while normal glass which have been mixed with potash etc. introducing phonons in the UV range and therefore is not transparent to these.
First, regarding your understanding of the ”phonon theory” (which in fact is not a theory), you are not very close.
Let me first explain phonons again. The atoms in a solid are not locked in a rigid pattern but can oscillate around their average position in the solid. Most solids have periodic arrays of atoms which forms a crystal lattice. Amorphous solids and glasses are exceptions. When a group of atoms oscillates in a synchronized way, it’s called lattice vibration. Different groups of atoms can oscillate (vibrate) with different frequencies which we call phonons. These vibrations are important for many properties of the solid, among them the transport of heat.
Now, to a “superlight” explanation of transparency:
Imagine the group of atoms as groups of people, each group dancing to their own rhythm in a room. Into the room comes a photon dancing in the rhythm of light (frequency of light). It tries to join a group in the dance (delayed), but they have a different rhythm, is rejected and sent to the next group in the same direction, and the same is repeated until the phonon escapes the room on the other side. If a group had the same rhythm as the phonon, the phonon would have joined the group in the dance and produced heat.
Since the lattice of atom arrangements in amorphous solids and glass is different from other solids, they do not have frequencies corresponding to the frequency of light; hence it is rejected after “test” and passed through.
When you mention that the phonons radiate, this is wrong, the amplitudes of the vibration is nowhere near a possibility to radiate outside the lattice.
As you mentioned, pure silica (SiO2) also called fused quarts will be transparent at 1200C (melting temperature is 2000C) while normal window glass melts at approx 1000C. Quarts glass will also be transparent to ultra violet light (no phonons corresponding to ultra violet light (200-380nm wavelength), while normal glass which have been mixed with potash etc. introducing phonons in the UV range and therefore is not transparent to these.
QUOTE (Tor+Nov 12 2005, 01:00 PM)
Quarts glass will also be transparent to ultra violet light (no phonons corresponding to ultra violet light (200-380nm wavelength), while normal glass which have been mixed with potash etc. introducing phonons in the UV range and therefore is not transparent to these.
Maybe better explanation should be, the heated glass is dissociating to the sodium a and polysilicate ions, whereas silica doesn't contains ions and retains nonconductive even at the high temperatures.
The movable ions (i.e. charged particles) are absorbed by the light waves are causing the nontransparency of glass at high temperatures. The same effect (the absorption easily movable electrons) is the reason of high absorption coefficient of metals even at the room temperature.
Maybe better explanation should be, the heated glass is dissociating to the sodium a and polysilicate ions, whereas silica doesn't contains ions and retains nonconductive even at the high temperatures.
The movable ions (i.e. charged particles) are absorbed by the light waves are causing the nontransparency of glass at high temperatures. The same effect (the absorption easily movable electrons) is the reason of high absorption coefficient of metals even at the room temperature.
Hi
Suppose we talk in unreal theory. A fairy tale.
Suppose the universe was an ocean of aether. or pre- matter, or prime matter,let's say. This
ocean that is our universe is, filled with particles that had ,say, in-facing magnetic fields, but were very mildly affected-attracted by magnetic fields .
Earth in this ocean drags amass of this ocean along with it trapped in the net of Earth's own
electro-magnetic field. This lobe of prime matter .....travels with us, let's say.
When light enters our lobe of prime matter ,it adjusts its speed to "light speed"WITHIN the
lobe, so when we measure it ,no matter how we do it , it always measures the same, 'cause
we're always in the traveling lobe of matter.
Just a story, guys.
Neala
Suppose we talk in unreal theory. A fairy tale.
Suppose the universe was an ocean of aether. or pre- matter, or prime matter,let's say. This
ocean that is our universe is, filled with particles that had ,say, in-facing magnetic fields, but were very mildly affected-attracted by magnetic fields .
Earth in this ocean drags amass of this ocean along with it trapped in the net of Earth's own
electro-magnetic field. This lobe of prime matter .....travels with us, let's say.
When light enters our lobe of prime matter ,it adjusts its speed to "light speed"WITHIN the
lobe, so when we measure it ,no matter how we do it , it always measures the same, 'cause
we're always in the traveling lobe of matter.
Just a story, guys.
Neala
Neala,
You realize that this is a forum for physics and science?
Your fairytale belongs to a fairytale forum, not here!
You realize that this is a forum for physics and science?
Your fairytale belongs to a fairytale forum, not here!
I think Tor was the Guest here
There is only one way light can really slow down. And that is by interacting with the EM field of matter.
At the tail end of wiki entry about something else (sorry I can't find it again) came the comment..
The interaction of the photon with the (whatever) effectively gives it the property of 'mass' and for this reason it can no longer travel at the speed of light.
I think I am convinced by this explanation.. I'd give Nick 10/10.
-C2.
QUOTE (Guest+Nov 3 2005, 10:28 PM)
This temporary absorbtion and retransmission is slowing down the photon, and the denser the solid, the slower is the light passing through.
Hi Tor,
Thanks for the clarification, sorry to have been so dense. I only introduced the heated glass to establish the absence of any radiative mechanism for visible light, after your clarification we seem to be in agreement that there is no 'conventional' radiation from the glass.
As already stated, my 'main problem' with transparent things is that the light comes out without (apparently) having picked up any trace of the thermal state of the thing it's just gone through. Your dancing photons and phonons are cute but (IMHO) a dead cert for superimposing the thermal state of the glass on the transmitted light, if nothing worse. Generally your suggestion seems to be that a photon can both create (if glass very cold) and/or interact (collide?) with a much lower energy state than itself (can you be more specific about energy level?), hang about a bit and then put everything back where it came from, then carry on as though nothing had happenned, no worries about the uncertainty principle, momentum and all that stuff. I'm not suggesting this is a theory - just checking.
Hi Zephir,
Yes, I quite agree. I think we're mostly OK with absorption, we might even be OK if the light went straight through - it's the damn delay that causes the problem, at least as far as I'm concerned.
Hi Troc,
-c2
Hi Tor,
Thanks for the clarification, sorry to have been so dense. I only introduced the heated glass to establish the absence of any radiative mechanism for visible light, after your clarification we seem to be in agreement that there is no 'conventional' radiation from the glass.
As already stated, my 'main problem' with transparent things is that the light comes out without (apparently) having picked up any trace of the thermal state of the thing it's just gone through. Your dancing photons and phonons are cute but (IMHO) a dead cert for superimposing the thermal state of the glass on the transmitted light, if nothing worse. Generally your suggestion seems to be that a photon can both create (if glass very cold) and/or interact (collide?) with a much lower energy state than itself (can you be more specific about energy level?), hang about a bit and then put everything back where it came from, then carry on as though nothing had happenned, no worries about the uncertainty principle, momentum and all that stuff. I'm not suggesting this is a theory - just checking.
Hi Zephir,
Yes, I quite agree. I think we're mostly OK with absorption, we might even be OK if the light went straight through - it's the damn delay that causes the problem, at least as far as I'm concerned.
Hi Troc,
-c2
Me thinks light is given its so called velocity, as a direct result of a higher dimensional export field which carries it away into the 4th dimension...please read my ridiculous BIG T.O.E concept.
Hi again C2, busy times, hence slow in answering,
A photon (whether particle or wave) cannot intrinsically be influenced by temperature, however, the phonons may be slightly influenced, and increased scattering of the photon may happen.
Here should be a picture but I have no idea how I can paste one in!!! If someone knows, please tell me (hope I do not need a web-host to do it!)
In order to visualize better what I am talking about, here is a diagram of SiO2 glass where [a] shows the real and imaginary part of the refractive index and [B] shows the extinction coefficient (absorption) of light (photon) by the phonon frequencies where you clearly see the transparency region between the IR phonons and the phonons above UV. In the visible spectra there are no phonons coinciding with the frequency of light, and the photon will therefore not be absorbed. But as I have said, the photon is temporarily absorbed but re-transmitted, hence the slowing down of speed through the glass.
Transfer of EM radiation (light) to the solid is in the form of couple, where the lattice vibration (phonon) produces an oscillating dipole moment which can be driven by the oscillating electric field (E) of the light if the frequencies coincides. The conservation of momentum is governed by the relationship between de Broglie’s partice/wave duality, from the photon and phonon momenta, where the photon momentum is P=h/lambda. The phonon momentum in the crystal is given by P=h/alfa, where alpha is the lattice constant for the unit cell. When lambda=alpha, the conservation of momentum is preserved between the photon and phonon. But the photon has a low momentum compared to that of the phonon so two or more photons are required to satisfy the conservation of momentum and produce a total absorption. If lambda is higher or lower than alpha, there can be no permanent absorption.
A photon (whether particle or wave) cannot intrinsically be influenced by temperature, however, the phonons may be slightly influenced, and increased scattering of the photon may happen.
Here should be a picture but I have no idea how I can paste one in!!! If someone knows, please tell me (hope I do not need a web-host to do it!)In order to visualize better what I am talking about, here is a diagram of SiO2 glass where [a] shows the real and imaginary part of the refractive index and [B] shows the extinction coefficient (absorption) of light (photon) by the phonon frequencies where you clearly see the transparency region between the IR phonons and the phonons above UV. In the visible spectra there are no phonons coinciding with the frequency of light, and the photon will therefore not be absorbed. But as I have said, the photon is temporarily absorbed but re-transmitted, hence the slowing down of speed through the glass.
Transfer of EM radiation (light) to the solid is in the form of couple, where the lattice vibration (phonon) produces an oscillating dipole moment which can be driven by the oscillating electric field (E) of the light if the frequencies coincides. The conservation of momentum is governed by the relationship between de Broglie’s partice/wave duality, from the photon and phonon momenta, where the photon momentum is P=h/lambda. The phonon momentum in the crystal is given by P=h/alfa, where alpha is the lattice constant for the unit cell. When lambda=alpha, the conservation of momentum is preserved between the photon and phonon. But the photon has a low momentum compared to that of the phonon so two or more photons are required to satisfy the conservation of momentum and produce a total absorption. If lambda is higher or lower than alpha, there can be no permanent absorption.
Hi Tor,
Much better! Thank you.
-----------------------------------------------------------------------------
Bit of a deviation but hopefully it helps..
Consider reflection from a mirror - optically flat but we know that in reality it is lumpy as hell. No single atom, electron, molecule, phonon or (probably) anything else you can suggest can 'know' the angle of the mirror without some form of communication with the rest of the surface (whistles? bells?.. well I hope you get the point). The inevitable (crazy) conclusion is that a single photon is reflected off a large area (in terms of atoms and molecules), possibly even the entire surface of the mirror. This gets worse, it is generally accepted that for both mirrors and lenses the maximum resolving power is determined by the diameter of the mirror or lens. When looking at distant stars we can easily get down to the level of counting photons - can single photons work out the diameter of a lens? :- apparently they can.
Lenses, as I'm sure we know, work by selectively slowing down light, exactly what we are looking at here, the point here being that it will not be possible to identify a 'single' interaction if there isn't one. It is possible that we may be looking at the right thing but possibly not in the right way.
---------------------------------------------------------------------------------
Please follow up my deviation - in the meantime I'll look at phonon/photon interaction. Would you mind emailing the picture to me? I'll email my email address to you until we work out how to get pictures onto the site.
-C2
Much better! Thank you.
-----------------------------------------------------------------------------
Bit of a deviation but hopefully it helps..
Consider reflection from a mirror - optically flat but we know that in reality it is lumpy as hell. No single atom, electron, molecule, phonon or (probably) anything else you can suggest can 'know' the angle of the mirror without some form of communication with the rest of the surface (whistles? bells?.. well I hope you get the point). The inevitable (crazy) conclusion is that a single photon is reflected off a large area (in terms of atoms and molecules), possibly even the entire surface of the mirror. This gets worse, it is generally accepted that for both mirrors and lenses the maximum resolving power is determined by the diameter of the mirror or lens. When looking at distant stars we can easily get down to the level of counting photons - can single photons work out the diameter of a lens? :- apparently they can.
Lenses, as I'm sure we know, work by selectively slowing down light, exactly what we are looking at here, the point here being that it will not be possible to identify a 'single' interaction if there isn't one. It is possible that we may be looking at the right thing but possibly not in the right way.
---------------------------------------------------------------------------------
Please follow up my deviation - in the meantime I'll look at phonon/photon interaction. Would you mind emailing the picture to me? I'll email my email address to you until we work out how to get pictures onto the site.
-C2
C2,
I understand where you are coming from and it is always the immediate thought by anyone who start to think microscopic; the surface is never totally flat, and if you think, like most, the atoms is like a ball and likewise the photon, it will be similar to snooker, you have to hit the other ball correctly to make it bounce in the desired angle. Well, this is not the way it is or happens with light reflection. (I am opening a can of worms here)
Let’s take your mirror which I suppose is a metallic solid, grinded to a very smooth surface. What you have done is to expose as many atoms as you could at the surface-line, and the more exposed, the better reflection.
As I pointed out earlier, metals have conduction electrons and free valence energy bands. When photons (light) hits the metallic surface, conduction electron absorbs and gains energy from the photon’s E-field causing it to temporarily change valence band. However, the conduction electron will drop back to its original valence band and re-emit the photon, keeping the transverse momentum, but with a 180 degree phase shift due to the negative charged conduction electron. (So there is the light “bouncing” for you).
Now to you resolving power, but first; never think of light as one photon!!
What determines the resolving power is the intensity/density of light over hit lens surface area, then the optical performance of the glass (scattering, refraction of colours etc. etc.)
I am not quite sure what you mean by “we may be looking at the right thing but possibly not in the right way”??
I understand where you are coming from and it is always the immediate thought by anyone who start to think microscopic; the surface is never totally flat, and if you think, like most, the atoms is like a ball and likewise the photon, it will be similar to snooker, you have to hit the other ball correctly to make it bounce in the desired angle. Well, this is not the way it is or happens with light reflection. (I am opening a can of worms here)
Let’s take your mirror which I suppose is a metallic solid, grinded to a very smooth surface. What you have done is to expose as many atoms as you could at the surface-line, and the more exposed, the better reflection.
As I pointed out earlier, metals have conduction electrons and free valence energy bands. When photons (light) hits the metallic surface, conduction electron absorbs and gains energy from the photon’s E-field causing it to temporarily change valence band. However, the conduction electron will drop back to its original valence band and re-emit the photon, keeping the transverse momentum, but with a 180 degree phase shift due to the negative charged conduction electron. (So there is the light “bouncing” for you).
Now to you resolving power, but first; never think of light as one photon!!
What determines the resolving power is the intensity/density of light over hit lens surface area, then the optical performance of the glass (scattering, refraction of colours etc. etc.)
I am not quite sure what you mean by “we may be looking at the right thing but possibly not in the right way”??
Hi Tor,
No pictures! Makes things a lot more difficult (has my emailing failed?)
Main theme first.. phonons as the mechanism for delay of light travelling through a solid..
To establish the nature of my type of phonons I quote from Wikipedia (search "phonons wiki" and find thermodynamic properties)
A crystal lattice at zero temperature lies in its ground state, and contains no phonons...(Note: the random motion of the atoms in the lattice is what we usually think of as heat) Because these phonons are generated by the temperature of the lattice, they are sometimes referred to as thermal phonons.
The point I feel is important here is not that phonons are influenced by temperature as that the population density for any chosen energy is very much influenced by temperature. Without the pictures I can't be sure but my impression so far is that every 'encounter' should result in a delay - small population gives small delay and vice versa - hence the theory should lead to a temperature dependence which I don't think happens in reality.
The other alternative (pictures please!!!) is that you suggest the photons are generating their own phonons. Please clarify.
Deviations from main theme..
I am programmed to deal with single photons first - I am that way.
"maximum resolving power is determined by the diameter of the mirror or lens" - check, check, check. It's a theoretical limitation - argue with the theory and I'll try to follow but don't dismiss it as just dirty lenses - there is a lot more to it than that.
Later addition-
Re: possibly not looking at it in the right way - just for example - if phonons could be shown to represent the 'entire structure' then your suggestion would look very promising. Same thing, different angle.
-C2.
No pictures! Makes things a lot more difficult (has my emailing failed?)
Main theme first.. phonons as the mechanism for delay of light travelling through a solid..
To establish the nature of my type of phonons I quote from Wikipedia (search "phonons wiki" and find thermodynamic properties)
A crystal lattice at zero temperature lies in its ground state, and contains no phonons...(Note: the random motion of the atoms in the lattice is what we usually think of as heat) Because these phonons are generated by the temperature of the lattice, they are sometimes referred to as thermal phonons.
The point I feel is important here is not that phonons are influenced by temperature as that the population density for any chosen energy is very much influenced by temperature. Without the pictures I can't be sure but my impression so far is that every 'encounter' should result in a delay - small population gives small delay and vice versa - hence the theory should lead to a temperature dependence which I don't think happens in reality.
The other alternative (pictures please!!!) is that you suggest the photons are generating their own phonons. Please clarify.
Deviations from main theme..
I am programmed to deal with single photons first - I am that way.
"maximum resolving power is determined by the diameter of the mirror or lens" - check, check, check. It's a theoretical limitation - argue with the theory and I'll try to follow but don't dismiss it as just dirty lenses - there is a lot more to it than that.
Later addition-
Re: possibly not looking at it in the right way - just for example - if phonons could be shown to represent the 'entire structure' then your suggestion would look very promising. Same thing, different angle.
-C2.
Hi C2,
Couple of things, it is very important to have some basic knowledge about physics before jumping to conclusions when reading about physical properties of phonons. Mind you, phonon is just another name for frequency due to its excitation. As you know we have frequencies from almost DC to cosmic rays and depending on the frequency interact different with solids, gases etc. What you are referring to are thermal phonons, which has nothing to do with our topic! Please do not confuse the issue.
Photons do NOT generate phonons. Phonons is the designation for vibration generated by the lattice of atoms in a solid as thoroughly explained.
If you want to understand the topic of light, you can not deal with a single photon! If anything experience light, it experiences a stream of photons, and solids deals with streams of photons, that is the only way it can know it’s the frequency of light.
As I have said, resolving power of a lens system is dependent on many different parameters, but the governing parameter is the amount of light/size of lens.
PS: I have sent the picture again via email, but there seems to be a problem at your end.
Reminder: If anyone can give an explanation on how to insert a picture in the post, I would be grateful!
Couple of things, it is very important to have some basic knowledge about physics before jumping to conclusions when reading about physical properties of phonons. Mind you, phonon is just another name for frequency due to its excitation. As you know we have frequencies from almost DC to cosmic rays and depending on the frequency interact different with solids, gases etc. What you are referring to are thermal phonons, which has nothing to do with our topic! Please do not confuse the issue.
Photons do NOT generate phonons. Phonons is the designation for vibration generated by the lattice of atoms in a solid as thoroughly explained.
If you want to understand the topic of light, you can not deal with a single photon! If anything experience light, it experiences a stream of photons, and solids deals with streams of photons, that is the only way it can know it’s the frequency of light.
As I have said, resolving power of a lens system is dependent on many different parameters, but the governing parameter is the amount of light/size of lens.
PS: I have sent the picture again via email, but there seems to be a problem at your end.
Reminder: If anyone can give an explanation on how to insert a picture in the post, I would be grateful!
We have pictures..
I've asked Tor to post up some background on phonons so we can establish some common ground.
I've asked Tor to post up some background on phonons so we can establish some common ground.
Hi Tor,
I think we've carried on long enough. My interest lies in the interaction of single photons with matter whereas yours lies in the interaction of many photons. The two approaches are divided by a common reality.
I withdraw any comment about the validity of your explanation because, as we have established, I don't understood it.
-C2
I think we've carried on long enough. My interest lies in the interaction of single photons with matter whereas yours lies in the interaction of many photons. The two approaches are divided by a common reality.
I withdraw any comment about the validity of your explanation because, as we have established, I don't understood it.
-C2
c2,
OK, I think we got off the topic anyway.
You will probably find it hard to explore the process of a single photon interacting with matter. Just be aware that a photon is still a hypotetic particle representing a quanta of an electromagnetic field.
Best of luck
Tor
OK, I think we got off the topic anyway.
You will probably find it hard to explore the process of a single photon interacting with matter. Just be aware that a photon is still a hypotetic particle representing a quanta of an electromagnetic field.
Best of luck
Tor
Nick writes:
"There is only one way light can really slow down. And that is by interacting with the EM field of matter."
Correct.
"I have been told it is absorption and emission of light in a medium that slows it down. But this doesn't actually CHANGE the SPEED of light."
For the full treatment, you'll need any undergraduate E-M textbook, but here's the gist.
The light doesn't just interact with the EM field of matter as if the two were separate. Light IS the electromagnetic wave rippling through the matter -- there's not a separate 'wave-that-was-in-the-vacuum', with separate induced fields on top of it. There is a single traveling electromagnetic field, which has at every point ONE electric field vector and (at right angles) one magnetic field vector.
First of all, we can ignore quantum effects to answer your question, and we can treat light as a classical electromagnetic wave.
The slowing-down of light in matter can be very accurately modeled by treating the matter as a simple bulk material. Let's simplify further and let the material be glass or plastic.
In a vacuum, a traveling electromagnetic wave follows a simple law of induction: (1) a changing magnetic field generates an electric field at right angles to the change, and (2) the changing electric field generates a magnetic field at right angles to the change. The two fields generate each other as they travel along. These laws of induction are most compactly summarized by Maxwell's Equations, which are basically all you need.
Vacuum is said to have an electric permeability and a magnetic permeability. That just means that if you put a voltage across a vacuum, the vacuum acts like a capacitor and stores' a certain amount of energy in an electric field. This energy 'really is' in the electric field.
Vacuum is also said to have a magnetic permeability. That just means that when you create a magnetic potential difference across a vacuum, it acts like a magnetic material and stores a certain amount of energy in a magnetic field. The magnetic energy 'really is' in the magnetic field.
Now suppose you have a microwave waveguide, a metal horn with a square aperture, facing into vacuum. y rapidly reversing the fields, you can 'shake loose' an electromagnetic wave into the vacuum. The electromagnetic wave can carry as much power as you can generate. If you freeze-frame the traveling electromagnetic wave, the total energy in the wave is equal to the total energy stored in the electric and magnetic fields.
Now suppose we cover the mouth of the microwave horn with resistive cloth called space cloth. If the space cloth has the exact same electrical resistivity as vacuum ( 288 ohms per square ), the electromagnetic wave gets soaked up by the space cloth just as if the wave had gone off into space and disappeared. If the electrical resistivity of the space cloth is DIFFERENT from the electrical impedance of vacuum, part of the microwave beam bounces off of the spacecloth with either a positive or negative phase.
It might seem odd that vacuum has an electrical impedance measured in ohms, but it does. Since you can push power through a vacuum with a microwave generator, vacuum must have a measurable impedance, corresponding to how much power you can push into a vacuum with a given amount of voltage ( a.k.a. electromotive force ).
A microwave beam behaves exactly like light in a vacuum, and travels at exactly the same velocity as light -- because a microwave beam IS light with a longer wavelength -- just as ultraviolet light and gamma rays are light with a shorter wavelength.
OK, that's how "light" behaves in a vacuum. Light is a traveling electromagnetic wave -- they're one in the same thing.
OK, let's step up the frequency to visible light and have it impact a sheet of plastic at a right angle. The light slows down. It's not a trick of path length -- the light actually slows down, in terms of the measured time to cross a unit distance. The plastic is said to have an index of refraction, measured by how much the light slows down. Refractive bending of light results from the light slowing down, because the wave fronts on either side of the boundary have to line up ).
Something else happens at the boundary -- a percentage of the wave bounces off the plastic, because there's an impedance mismatch -- the measured electrical impedance is different inside the plastic. The reflected wave tells you that the wave-in-vacuum doesn't just keep going 'as is' into the plastic. The plastic can not only change the velocity of the wave. but can cause part of the wave to rebound, which is why plastic looks shiny.
Inside the plastic, the movement of the electromagnetic wave is still governed by the laws of induction -- a changing electric field generates a magnetic field, and a changing magnetic field generates a magenetic field.
The difference is that each atom in the plastic has a positive nucleus and negative electrons. The electrons can move slightly in response to the electric field of the light. The slight movement of charge is caused a displacement current. The displacement of charge causes an induced electric field and an induced magnetic field.
Here's where it gets interesting. Maxwell's Equations don't just work in a vacuum -- they also work around electric currents. You can in principle solve Maxwell's equations for any arbitrarily complex configuration of moving charges and fields. Here we get lucky -- the solution is still a simple traveling ( plane ) wave.
So why is it slower?
The reason, in a nutshell, is that the electrons take time to respond to the electric field of the incoming light -- there's a phase delay. When the electrons do respond, they move so as to absorb energy from the incoming electric field ( like charging up a capacitor ). The induced electric field is roughly OPPOSITE to the incoming electric field. In other words, the original wave can be thought of as constantly being absorbed and re-emitted by the more sluggish charged matter.
Let's switch to an even simpler question, why does a TV wave slow down in plastic ribbon wire or coaxial cable? Suppose we start without the plastic, and just have two parallel conductors in a vacuum. If you hook the parallel wires up to a TV signal, the signal travels at the speed of light in vacuum -- because when you apply Maxwell's Equations to the electric and magnetic fields in the space around the wire, that space is vacuum, so you plug in the electric and magnetic permeability of vacuum.
Another way of solving the parallel-wire problem ( which comes out the same )
is to treat the wire as a series of small series inductors ( representing the inductance of wire in vacuum ) bridged by small capacitors between the wires, representing the distributed capacitance of two wires close together in vacuum. That's also a model for a delay line -- because the larger the capacitors are, the longer it takes them to charge up, causing a phase delay.
The end result is the same -- a TV wave travels along parallel wires at the speed of light if there's nothing between the wires.
If we add plastic insulation between the wires, the inductance is essentially unchanged, but the capacitance between the wires is markedly increased, and the signal slows down by 20 % to a third. This slowdown helps keep the signal from leaking out of the ribbon wire, because the speed and wavelength no longer match up with vacuum.
Again, to properly understand this you'll need to experiment with Maxwell's Equations, which are differential equations -- they express the electric field AT ANY POINT in terms of the change-of-magnetic field at that same point, and vice versa. You'll also need some calculus to see why the solutions are traveling plane waves of a particular velocity. That may seem like a lot of work, but it gives you a powerfully generalized understanding of how waves work, and why most faster-than-light schemes don't work.
Quantum mechanics wasn't needed in the above, because the quantum mechanics is folded into the index of refraction and dielectric constant, which for most transparent materials are fairly constant across the visible spectrum. ( Up in the ultraviolet range, glass and plastic have strong quantum mechanical resonances that can cause two slightly different frequencies of light to behave very differently ). However at ordinary visible-light wavelengths, the only quatum-nechanical effect is a slight variation of refractive index with frequency, which causes a prism to spread colors of light.
"There is only one way light can really slow down. And that is by interacting with the EM field of matter."
Correct.
"I have been told it is absorption and emission of light in a medium that slows it down. But this doesn't actually CHANGE the SPEED of light."
For the full treatment, you'll need any undergraduate E-M textbook, but here's the gist.
The light doesn't just interact with the EM field of matter as if the two were separate. Light IS the electromagnetic wave rippling through the matter -- there's not a separate 'wave-that-was-in-the-vacuum', with separate induced fields on top of it. There is a single traveling electromagnetic field, which has at every point ONE electric field vector and (at right angles) one magnetic field vector.
First of all, we can ignore quantum effects to answer your question, and we can treat light as a classical electromagnetic wave.
The slowing-down of light in matter can be very accurately modeled by treating the matter as a simple bulk material. Let's simplify further and let the material be glass or plastic.
In a vacuum, a traveling electromagnetic wave follows a simple law of induction: (1) a changing magnetic field generates an electric field at right angles to the change, and (2) the changing electric field generates a magnetic field at right angles to the change. The two fields generate each other as they travel along. These laws of induction are most compactly summarized by Maxwell's Equations, which are basically all you need.
Vacuum is said to have an electric permeability and a magnetic permeability. That just means that if you put a voltage across a vacuum, the vacuum acts like a capacitor and stores' a certain amount of energy in an electric field. This energy 'really is' in the electric field.
Vacuum is also said to have a magnetic permeability. That just means that when you create a magnetic potential difference across a vacuum, it acts like a magnetic material and stores a certain amount of energy in a magnetic field. The magnetic energy 'really is' in the magnetic field.
Now suppose you have a microwave waveguide, a metal horn with a square aperture, facing into vacuum. y rapidly reversing the fields, you can 'shake loose' an electromagnetic wave into the vacuum. The electromagnetic wave can carry as much power as you can generate. If you freeze-frame the traveling electromagnetic wave, the total energy in the wave is equal to the total energy stored in the electric and magnetic fields.
Now suppose we cover the mouth of the microwave horn with resistive cloth called space cloth. If the space cloth has the exact same electrical resistivity as vacuum ( 288 ohms per square ), the electromagnetic wave gets soaked up by the space cloth just as if the wave had gone off into space and disappeared. If the electrical resistivity of the space cloth is DIFFERENT from the electrical impedance of vacuum, part of the microwave beam bounces off of the spacecloth with either a positive or negative phase.
It might seem odd that vacuum has an electrical impedance measured in ohms, but it does. Since you can push power through a vacuum with a microwave generator, vacuum must have a measurable impedance, corresponding to how much power you can push into a vacuum with a given amount of voltage ( a.k.a. electromotive force ).
A microwave beam behaves exactly like light in a vacuum, and travels at exactly the same velocity as light -- because a microwave beam IS light with a longer wavelength -- just as ultraviolet light and gamma rays are light with a shorter wavelength.
OK, that's how "light" behaves in a vacuum. Light is a traveling electromagnetic wave -- they're one in the same thing.
OK, let's step up the frequency to visible light and have it impact a sheet of plastic at a right angle. The light slows down. It's not a trick of path length -- the light actually slows down, in terms of the measured time to cross a unit distance. The plastic is said to have an index of refraction, measured by how much the light slows down. Refractive bending of light results from the light slowing down, because the wave fronts on either side of the boundary have to line up ).
Something else happens at the boundary -- a percentage of the wave bounces off the plastic, because there's an impedance mismatch -- the measured electrical impedance is different inside the plastic. The reflected wave tells you that the wave-in-vacuum doesn't just keep going 'as is' into the plastic. The plastic can not only change the velocity of the wave. but can cause part of the wave to rebound, which is why plastic looks shiny.
Inside the plastic, the movement of the electromagnetic wave is still governed by the laws of induction -- a changing electric field generates a magnetic field, and a changing magnetic field generates a magenetic field.
The difference is that each atom in the plastic has a positive nucleus and negative electrons. The electrons can move slightly in response to the electric field of the light. The slight movement of charge is caused a displacement current. The displacement of charge causes an induced electric field and an induced magnetic field.
Here's where it gets interesting. Maxwell's Equations don't just work in a vacuum -- they also work around electric currents. You can in principle solve Maxwell's equations for any arbitrarily complex configuration of moving charges and fields. Here we get lucky -- the solution is still a simple traveling ( plane ) wave.
So why is it slower?
The reason, in a nutshell, is that the electrons take time to respond to the electric field of the incoming light -- there's a phase delay. When the electrons do respond, they move so as to absorb energy from the incoming electric field ( like charging up a capacitor ). The induced electric field is roughly OPPOSITE to the incoming electric field. In other words, the original wave can be thought of as constantly being absorbed and re-emitted by the more sluggish charged matter.
Let's switch to an even simpler question, why does a TV wave slow down in plastic ribbon wire or coaxial cable? Suppose we start without the plastic, and just have two parallel conductors in a vacuum. If you hook the parallel wires up to a TV signal, the signal travels at the speed of light in vacuum -- because when you apply Maxwell's Equations to the electric and magnetic fields in the space around the wire, that space is vacuum, so you plug in the electric and magnetic permeability of vacuum.
Another way of solving the parallel-wire problem ( which comes out the same )
is to treat the wire as a series of small series inductors ( representing the inductance of wire in vacuum ) bridged by small capacitors between the wires, representing the distributed capacitance of two wires close together in vacuum. That's also a model for a delay line -- because the larger the capacitors are, the longer it takes them to charge up, causing a phase delay.
The end result is the same -- a TV wave travels along parallel wires at the speed of light if there's nothing between the wires.
If we add plastic insulation between the wires, the inductance is essentially unchanged, but the capacitance between the wires is markedly increased, and the signal slows down by 20 % to a third. This slowdown helps keep the signal from leaking out of the ribbon wire, because the speed and wavelength no longer match up with vacuum.
Again, to properly understand this you'll need to experiment with Maxwell's Equations, which are differential equations -- they express the electric field AT ANY POINT in terms of the change-of-magnetic field at that same point, and vice versa. You'll also need some calculus to see why the solutions are traveling plane waves of a particular velocity. That may seem like a lot of work, but it gives you a powerfully generalized understanding of how waves work, and why most faster-than-light schemes don't work.
Quantum mechanics wasn't needed in the above, because the quantum mechanics is folded into the index of refraction and dielectric constant, which for most transparent materials are fairly constant across the visible spectrum. ( Up in the ultraviolet range, glass and plastic have strong quantum mechanical resonances that can cause two slightly different frequencies of light to behave very differently ). However at ordinary visible-light wavelengths, the only quatum-nechanical effect is a slight variation of refractive index with frequency, which causes a prism to spread colors of light.
QUOTE (Nick .. opening post+)
There is only one way light can really slow down. And that is by interacting with the EM field of matter.
At the tail end of wiki entry about something else (sorry I can't find it again) came the comment..
The interaction of the photon with the (whatever) effectively gives it the property of 'mass' and for this reason it can no longer travel at the speed of light.
I think I am convinced by this explanation.. I'd give Nick 10/10.
-C2.
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