Refractive indices for aromatic molecules are higher than those of saturated hydrocarbons.

Why is the refractive index of Benzene(n=1.50112 @ 20 degrees celcius) greater than the refractive index of Cyclohexane(n=1.4290 @ 20 degrees celcius) ?

Why does light travel more slowly in a liquid (for water n=1.333)?

C_E,



Based on the opening statement, and your board name, I'll assume that you know this already, but for the sake of other readers, would first clarify something.


In the context of your 2 questions, it appears as though you are saying that "..travel more slowly in a liquid (for water n=1.333)" is in comparison to your other examples - Benzene(n=1.50112..) and Cyclohexane(n=1.4290..). This would be incorrect.


As the refractive index (RI) gets smaller, the velocity gets larger. So, at an RI of " 1 ", the velocity = 299,792,458. Since the RI is frequency dependent, this leads to my first question:

What kind of "light" are you referring to? If a LASER, then what kind of LASER, at what frequency, and intensity (power)?


The reason is, since you've eliminated temperature variation, we've just got "geometry" left. The lattice, or structure of the bonds and locations of absorbers/emitters can also add to this picture. If we get a specific kind of "internal reflection", the optical Kerr effect can be produced, and cause an intensity dependent variation to the RI. (IDRI)


In your "faster" example, the "light" was likely subject to self focusing, and self phase modulation (SPM) due to the IDRI. It (the sample) was more "transparent" than the other, for specific reasons due to the kind of light used. While in the "slower" example, the light had a pulse distortion that changed the phase velocity. In this case, it is phase velocity that determines the rate of energy exchange, and not the usual group velocity/envelope.


In liquids there is random motion involved, and much more dispersion. The dispersion prevents the focusing and SPM. The molecules are more "spread out", so the same principles apply, but to a lessor extent.


This is a very simplified explanation, hopefully not too much so.



regards,

T.Roc

Thank you sir!! I eventually figure it out.

They were in fact different questions..sorry for not making it clearer

I would also like to add that refractive index of a liquid is a function of its density and hence temperature. Also the bonding system in benzene also plays a role..Pi bonds compare to sigma bonds

We're talking about materials with no magnetic permeability, so the refractive index comes only from their electrical permittivity.

At optical frequencies, the movement induced by electromagnetic waves (light) on electrons in normal molecules (small ones with electrons rather firmly located) is limited by the stiffness of the electron's orbitals rather than the electron's mass.

Please keep in mind that this view is too simplified and QM should be applied. But at least it gives some intuitive representation. For instance, it explains why the refractive index rises with frequency.

Now, if you give a bigger orbital for some electrons, like in an aromatic ring, these electrons move more easily, giving more permittivity and refractive index.

Going further, you may build a long molecule having delocalized electrons, similar to beta-carotene, and put reservoirs (aromatic rings) at the ends. As the molecule becomes long enough, its electrons move a lot when illuminated; And if their stiffness is low enough, the combination with their masses makes them resonate at optical frequencies.

Congratulations, you've made a dye.

Adjust the stiffness through the length of the molecule to choose the colour.

Light slower in liquids:
- Because speed in vacuum is the maximum speed
- Because matter contains charged particles that can be moved somewhat

Temperature:
The effect on refractive index is faster than the density variation, so look rather at shocks between molecules.
If permittivity is made by charged atoms displacement, the reason is clear, as shocks prevent the quiet displacement of atoms in the electric field. However, such displacements act rather in the infrared spectrum of the molecule.

Great!! Thank you Enthalpy