Sunday, February 3, 2019

special relativity - What happens when a photon hits a mirror?


When a photon of light hits a mirror does the exact same photon of light bounce back or is it absorbed then one with the same properties emitted? If the same one is bounced back does it's velocity take all values on $[-c,c]$ or does it just jump from $c$ to $-c$ when it hits the mirror?


Or, is the phenomenon of a mirror better explained using a wave analogy? If so, what is this explanation?



Answer



How do mirrors work? is closely related to your question, if not a precise duplicate.


We normally think of photon scattering as absorbing the original photon and emitting a new one with a different momentum, so in your example of the mirror the incoming photon interacts with the free electrons in the metal and is absorbed. The oscillations of the free electrons then emit a new photon headed out from the mirror. Unlike e.g. electrons, photon number isn't conserved and photons can be created and destroyed whenever they interact.


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