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Photon energy 
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#1
Dec1504, 11:07 AM

P: 79

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
Dec1504, 02:41 PM

Mentor
P: 11,780

Of course, they might be doing something completely different, but you didn't provide any context, so it's hard to tell. [tex]E^2 = \sqrt {p^2 c^2 + m^2 c^4}[/tex] Set [tex]m = 0[/tex] and [tex]E = hf[/tex] for a photon. Also for a photon you have good old [tex]c = f \lambda[/tex]. 


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