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Why do mirrors reflect so much light?

  1. Feb 2, 2013 #1
    Hello

    Can someone explain what it is about typical household mirrors (eg. for grooming use) that allows them to reflect so much light (I think they generally reflect about 98% or so?).
    ie. glass only reflects roughly 4% - so what is the reflective agent in typical mirrors?
     
  2. jcsd
  3. Feb 2, 2013 #2

    mathman

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    The glass has a coating of a metal (usually silver) which reflects.
     
  4. Feb 2, 2013 #3
    Thanks - presumably the silver must be polished to reflect so much - what does polishing do? - does it create lots of thing layers which lead to constructive thin film interference? - if that is the case, it is surprising that the reflected image has such a high colour fidelity... ie. thin film interference is wavelength dependent so one would expect to see bands of colours produced such as in an oil slick... .
     
  5. Feb 2, 2013 #4
    additionally; what is the function that the glass plays? Why not just have a silvered surface and not place a layer of glass over it?
     
  6. Feb 2, 2013 #5
    No, it's not thin layer interference.
    Metals reflect visible light. No matter if they are polished or not. A smooth (polished) surface is necessary only to have specular reflection. This means basically that when a beam of parallel rays falls on the mirror, all reflected rays are in the same direction.
    You can make a mirror from a slab of polished metal (no glass). This is what they did in ancient times.
    The cheap modern mirrors are made by coating the glass with a a thin layer of aluminum. (I think is aluminum, not sure).
    This makes the mirror cheap and ensure that the metal layer is protected from the oxidation so is good for a long time.

    However good (and expensive) lab mirrors are made with the reflective metal layer on top of the glass so the light does not have to go through glass.
     
  7. Feb 2, 2013 #6

    cjl

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    It protects the metal, and keeps the metal surface from getting damaged or gaining any imperfections. Really high grade mirrors (such as would be used in lab settings or in good telescopes) have the metal side on the front, which works better, but it also makes them more delicate, since the glass is no longer protecting the surface.
     
  8. Feb 3, 2013 #7

    sophiecentaur

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    Early mirrors used mercury ('quicksilver') as the 'silvering'. As a liquid, mercury is structurally very weak and the only way to get a usable layer of mercury would be to put it at the back of the protective glass, with a strong backing to hold it there. Also, the glass keeps the air away so it stops the shiny surface from tarnishing. Old mercury silvered mirrors are very poor mirrors - a sort of yellowy colour. They may have been better when the were new, though.
     
  9. Feb 3, 2013 #8
    Mercury was in use to make liquid silver amalgam, which after mirror covering vaporises leaving pure silver on the glass.
    Modern mirrors covered with aluminium. By blowing up thin aluminium wire in high vacuum.

    The only reason for glass, i think, is to create flat surface on wavelength scale. Many other materials, has polycrystall structure, and therefore sharp surface. Proper polishing is difficult task, because some microcrystalls prefer to leave surface entirely.
     
  10. Feb 3, 2013 #9

    davenn

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    as has been partly answered the glass is just a holder for the thick aluminium deposit

    one electronics industry I used to work in with the older style overhead projectors ( the ones that used a sheet of acetate) the mirror in these units has the reflective surface on the front side ... this was to stop the glass fron getting too hot ie... if the reflective surface was on the rear side then the glass would absorb a lot of the heat from the high wattage lamp. With the reflective surface on the front, the much of the heat and light was reflected before it could enter the glass

    which is why (particularly telescopes) the mirrored surface is usually covered in a protective layer/coating, often fluorite based

    Dave
     
  11. Feb 4, 2013 #10
    Even earlier, Egyptians used polished copper or bronze plates as mirrors. A brief review of mirrors history here:

    http://www.designboom.com/history/mirror.html [Broken]
     
    Last edited by a moderator: May 6, 2017
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