# The mass of photon

1. May 17, 2007

### physicsatiztech

"the mass of photon"

I definitely know that photons don't have any mass if it is moving with it's well-known speed. However, it has its rest mass. So my question is

The energy of photons depend on their frequencies, however the freq. may differ from photon to photon. So I think their rest mass should also differ from photon to photon...

Am I right, or .....?
thanks a lot

2. May 17, 2007

### Staff: Mentor

Photons are massless--they have zero rest mass.

3. May 17, 2007

### pmb_phy

And that speed is always a constant (i.e. has the same value irrespective of the frame of reference) having the value c. The rest mass of any particle which moves with speed = c will have zero rest mass.
You're not quite right in the sense that you're speaking about two different definitions of mass as if they were one. The value E = pc for a photon means that its rest mass is zero but that its inertial mass is not zero.

Pete

4. May 18, 2007

### FUNKER

I asked this same question many moons ago
MINT!