Mirrors & Prisms: Reflecting Wavelengths

[sjon]

what is better ? a prism or a mirror (to reflect as much as possible wavelengths)

 

[tiny-tim]

Welcome to PF!

what is better ? a prism or a mirror (to reflect as much as possible wavelengths)

Hi sjon! Welcome to PF!

A mirror affects all wavelengths the same, but a prism affects them differently.

 

[Redbelly98]

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:

           | \
------>----+--\
           |  |\
           |  |/  (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.

EDIT:
I found a better figure than mine for showing how a Porro prism works:
http://en.wikipedia.org/wiki/Porro_prism