# One question about a photon and another about light

How can a photon have a momentum p, which if I'm not wrong is equal to mass X speed, when the photon doesn't have a rest mass???

what does it mean to say that light is an electromagnetic wave?

thnx

## Answers and Replies

Yes, the photon has "relativistic mass" or inertia, but "rest mass" is a completely different concept.

To say light is an EM wave? It means familiar (e.g., coloured) light is made out of the same stuff as radio waves (which are produced by shaking magnets, for example), and that (rather than always going in straight rays) light can be demonstrated to refract or interfere with itself.

thanx for your reply, could u please elaborate on relativistic mass? if u have time that is :)

So not all photons travel at the speed of light c, cause that would not make any sence unless I missed something

Relativistic mass is a misleading and unncecessary concept, it is much simpler to say that the photon has no mass. And of course it always travels at c in a vacuum.

Photons and classical EM waves carry a momentum that isn't given by p = mv, but they do transfer energy and momentum. The momentum of a massless particle is related to its energy by E = pc.

By the way, sunburn is caused by the momentum that EM waves carry.

Doc Al
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How can a photon have a momentum p, which if I'm not wrong is equal to mass X speed, when the photon doesn't have a rest mass???
That's where you're wrong: momentum only equals mass X speed for particles with non-zero mass traveling at (relatively) slow speeds. As confinement explained, massless particles (like photons) do carry momentum but it's not given by mv.