Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

Is there a type of glass, mirror or lens that allows only lights of amplitude equal to or greater than a fixed value to pass/reflect through them?

share|cite|improve this question
up vote 15 down vote accepted

Yes, there are. Such materials are called "saturable absorbers," and are (or at least, have been) used as switches in some laser designs. The one I recall is a nickel acetate dye, although there are others. Basically, the molecules absorb single photons at the laser wavelength, but when the intensity is great enough that two photons are absorbed simultaneously (or precisely, within the decay time of the single-photon excited state), the excited molecule no longer absorbs at that wavelength and the high-power beam is transmitted. This was done in, e.g., early hand-held laser rangefinders to produce a higher-power, shorter length output pulse.

Interestingly enough, there are also saturable transmitters. Essentially this is what's layered onto eyeglasses to produce those lenses which are sunglasses in bright light but clear in dim light.

share|cite|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.