Mirrors & Prisms: Reflecting Wavelengths

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SUMMARY

The discussion centers on the comparative effectiveness of Porro prisms versus mirrors in reflecting wavelengths. A Porro prism utilizes total internal reflection, achieving 100% reflection efficiency, while mirrors exhibit wavelength dependence due to their reflective coatings. The choice between the two depends on the feasibility of creating uniform anti-reflection or high-reflection coatings across a broad spectrum. Binoculars, which employ Porro prisms, suggest that they may offer superior performance or cost-effectiveness compared to mirrors.

PREREQUISITES
  • Understanding of total internal reflection principles
  • Familiarity with optical coatings, specifically anti-reflection and high-reflection coatings
  • Knowledge of Porro prism design and functionality
  • Basic concepts of wavelength dependence in optics
NEXT STEPS
  • Research the design and applications of Porro prisms in optical devices
  • Explore the principles of total internal reflection in depth
  • Investigate the manufacturing processes for anti-reflection and high-reflection coatings
  • Study the performance metrics of mirrors versus prisms in various optical systems
USEFUL FOR

Optical engineers, physicists, and anyone involved in the design of optical instruments, particularly those comparing the performance of prisms and mirrors in wavelength reflection.

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

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

Hi sjon! Welcome to PF! :smile:

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:

Code: 
           | \
------>----+--\
           |  |\
           |  |/  (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
 
Last edited:

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