9th March 2006 - 07:05 PM
just out of curiosity, how are mirrors- with their smooth surfaces- made?
10th March 2006 - 09:26 AM
Early mirrors were created by simply polishing a suitable substance until it became highly reflective. Neolithic mirrors have been discovered, made by grinding down obsidian rocks and polishing them to an incredible sheen.
To make a Neolithic mirror, one would need to find the right stone, ideally obsidian. A rougher stone is used to grind the base stone down to a flat surface on one side. After a flat surface is achieved, a finer grinding stone and clay slip can be used to polish the stone to a fully reflective sheen. Extremely fine abrasives, such as ash, can help further. After a substantial amount of time and effort, one will have created a very primitive form of mirror.
Modern mirrors, however, are made using an entirely different process. By taking liquid metals and allowing them to condense on a sheet of glass, one can get a surface far more reflective than anything achieved by polish. Making a mirror can be done at home with only a few supplies easily acquired at a local chemistry shop.
With pure silver nitrate, distilled water, and ammonia, one can make a mirror virtually indistinguishable from those purchased at a store. The process involves dissolving a small amount of silver nitrate in distilled water, then adding diluted ammonia until the mixture goes through distinct chemical changes. A second mixture is made using silver nitrate and Rochelle salts. This mixture must be boiled and filtered. By pouring these mixtures on to a very clean piece of glass sufficiently heated to the proper temperature, the silver will precipitate and form an even coating on the glass. After drying, one can coat the back of the silver with a solid paint to help prevent degradation of the silver. The result is a fully functional mirror.
Commercial mirrors are manufactured in more or less the same manner as is described above, although materials such as aluminum might be used instead of silver.
12th March 2006 - 04:45 PM
15th March 2006 - 06:17 AM
thanks! i didn't know that..
14th May 2006 - 11:24 AM
Thanks I really did not know this. You have explained it in such a wonderful way. Again a very big thanks.
14th May 2006 - 03:21 PM
Large, single piece mirrors for telescopes are often made by placing many tons of glass into a rotating ceramic mold which is placed into a giant furnace, such that the mirror will have a natural concave shape. Once the shape has been set (the faster the spin, the more concave the mirror), the whole thing is allowed to cool for many months (still spinning, to maintain the shape) before being polished.
Indeed, some telescopes actually use a disc of spinning liquid (like mercury) instead of an actual solid mirror. This allows the user to change the shape of the mirror on the fly - by simply adjusting the rate of rotation.
15th May 2006 - 05:04 AM
Besides the metal coating on the back (or for telescope mirrors, the front) side of the glass, there is also the question of how the glass is made perfectly flat in the first place. In fact, this is a rather recent process. If you look at windows in old buildings ( over 100 years old), they often have rather ripply glass in them. This is because glass used to be poured into windowpane molds and cooled in them like making flat ice cubes. They would be perfectly flat on the top side, but only as flat as the mold on the bottom, so they were slightly ripply when you looked through them. This was also the reason that old fashioned windows were made by framing several small panes of glass together.
The more modern method is pretty clever, and takes advantage of the fact that people already know that the top side of the glass turned out perfectly flat in the old method. The new method is to make a pool of mercury (or any material with a melting point lower than that of glass, but mercury is best) and pouring the melted glass onto the pool. Since the mercury is much heavier, the glass floats, and since mercury and glass are both liquids, their top surfaces are each perfectly flat. But the top surface of the mercury is the bottom surface of the glass, so the glass is perfectly flat on both the top and the bottom surfaces. This method makes nice non-ripply glass, and can also be used to make very large glass panes.
This modern process is also the only one capable of making very large panes of glass that are nice and flat, which is why old houses did not have "picture windows." Today, large panes of glass are so common that hardly anybody thinks about the fact that they are a pretty recent invention. In fact, in previous centuries, glass mirrors were frighteningly expensive, because they were so hard to make large and flat, and nobody wants a ripply mirror. I was watching an old movie just yesterday about Queen Elizabeth the First, and in the movie she threw a fit about being old and ugly, and went about smashing all the mirrors in her room. I had to laugh, because those mirrors would have cost more than her throne, her jewels, and her crown.
There, that was some more historical information for you!
18th August 2006 - 02:44 PM
Is it possible to create mirror using thin metal coating (micro/nano technology) and grinding it using strong laser beam?
Is it possible to create giant mirrors for telescope using this method?