# Photon energy

1. Dec 15, 2004

### repugno

On a couple of webisites I have seen that they have equated E=mc^2 with E=hf .... i.e hf=mc^2

But how can this be done when E=hf is describing the energy of a photon and E=mc^2 isnt?

Also, can someone provide me with a derivation of de Broglie’s theorem ?

Wavelength = h/momentum
thank you

2. Dec 15, 2004

### Staff: Mentor

I bet they're deriving the "relativistic mass" of a photon. I don't consider the concept of "relativistic mass" to be very useful in general, let alone for a photon. But some people seem to like the idea.

Of course, they might be doing something completely different, but you didn't provide any context, so it's hard to tell.

Start with the general relationship between energy, momentum and invariant mass (not "relativistic mass"):

$$E^2 = \sqrt {p^2 c^2 + m^2 c^4}$$

Set $$m = 0$$ and $$E = hf$$ for a photon. Also for a photon you have good old $$c = f \lambda$$.