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Mirrors & Prisms

  1. Oct 26, 2008 #1
    what is better ? a prism or a mirror (to reflect as much as possible wavelengths)
  2. jcsd
  3. Oct 27, 2008 #2


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    Welcome to PF!

    Hi sjon! Welcome to PF! :smile:

    A mirror affects all wavelengths the same, but a prism affects them differently.
  4. Oct 27, 2008 #3


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    sjon, if you mean a Porro prism, I think it is at least as good (if not better) than a mirror.

    A Porro prism relies on total internal reflection and normally-incident transmission:

    Code (Text):

               | \
               |  |\
               |  |/  (90-degree angle in prism)
               | /
    The reflections are 100%, since they are total-internal reflections. Any wavelength dependence would be in the anti-reflection coating on the front (left side) face.

    Compare that to a mirror, where the wavelength dependence arises from the mirror's reflective coating.

    So, the question of which is better depends on whether it's easier to make an anti-reflection or a high-reflection coating uniform AND close to 100% over a wide range of wavelengths. Anybody else have any thoughts on this?

    I am thinking that since binoculars use Porro prisms, they are either better than a mirror or much cheaper than a mirror of the same performance.

    I found a better figure than mine for showing how a Porro prism works:
    Last edited: Oct 27, 2008
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