[Ronald Berman]

Ronald Berman  asked the Naked Scientists:
   
A ball stops dead for a moment when it bounces off a wall.

How can the speed of light remain constant when it reflects off a mirror?

Thank you.

Ronnie B.

What do you think?

[graham.d]

This is a good question. You can think of this in a number of ways because of the nature of electromagnetic radiation being represented either as a photon (particle) or a wave. If you think of it as a wave, the act of reflection is really to do with electrical conduction in the surface of the reflector reacting to the changes of the electric field produced by the incident em wave. In a perfect conductor the wave would cause the electrons (these are the likely carriers) in the surface to move so as to cause the wave to be re-emitted (if I remember, in anti-phase) to the incident wave. In practice, conductors are not perfect and the conduction is not all exactly at the surface, so there is some energy loss. Quantisation means that this takes the form that there is some absorption. In terms of photons, some are absorbed and some are reflected. Other imperfections can produce scattering also.

So really you could think of it as the speed remaining constant and, in photon terms, a "new" photon being created going in the other direction.

It also should be noted that the speed of light that is regarded as constant is the speed of light in a vacuum, and more exactly, it is really the group velocity that is constant.