Register to reply

Photon energy

by repugno
Tags: energy, photon
Share this thread:
repugno
#1
Dec15-04, 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
Phys.Org News Partner Physics news on Phys.org
Physical constant is constant even in strong gravitational fields
Physicists provide new insights into the world of quantum materials
Nuclear spins control current in plastic LED: Step toward quantum computing, spintronic memory, better displays
jtbell
#2
Dec15-04, 02:41 PM
Mentor
jtbell's Avatar
P: 11,875
Quote 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"):

[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].


Register to reply

Related Discussions
Dissociation of H2O due to Photon energy. Advanced Physics Homework 5
Energy of the photon Introductory Physics Homework 4
Help - energy of a photon :-/ Introductory Physics Homework 2
Help - energy of a photon Introductory Physics Homework 1
Energy of a photon Introductory Physics Homework 9