what is better ? a prism or a mirror (to reflect as much as possible wavelengths)
Welcome to PF!
Hi sjon! Welcome to PF!
A mirror affects all wavelengths the same, but a prism affects them differently.
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.
I found a better figure than mine for showing how a Porro prism works:
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