## Photon energy

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

 Also, can someone provide me with a derivation of de Broglie’s theorem ? Wavelength = h/momentum
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 $$c = f \lambda$$.