Light/momentum question?

Question

Would a light beam that travels say 10m take less time than a light beam that travels 5m to a mirror gets reflected and travels 5m in the opposite direction?

I know light's velocity is constant but what happens at the point of reflection at the mirror at the photonic scale? Do the photons slow down to 0m/s then speed up in the other direction?

Answer 1

Since the light beam actually transfers momentum to the reflector, I believe you can "consider" that the photons have slowed down and reversed direction. But that is not what "really" happens. The time it takes for 10m in one direction and two 5m paths is the same, as the velocities are identical in both directions. The momentum of a photon is not related to its velocity (as it is in physical objects); it is a function of its wavelength only.

Answer 2

The intensity of light will decrease exponentially as it enters the metal. On the photonic scale, individual photons will make it past individual layers of the metal with a particular probability, but when each photon reflects individually, it will change its momentum instantaneously.

The time delay associated with reflections is most easily seen during total internal reflections within a dielectric. Here it is known as the Goos-Hanchen effect.

Answer 3

There will indeed be a delay in reflection at the mirror. How long the mirror takes to reflect depends on the electorn response, and this varies for different materials. But the effect is tiny.

Answer 4

Good question but they can't slow down as they only exist at the speed of light - if they slowed they would disappear