# Mass of a Photon

1. Nov 10, 2015

### Phys_Boi

If the energy of a photon: Ephoton = hν, where h = Planck's constant,
and E = mc2, then does hv = mc2

And if so, if c=vλ: Ephoton = hc/λ = mc2

Thus mass: m = h/λc ?

2. Nov 10, 2015

### Khashishi

$E=mc^2$ only applies to massive particles at rest, not photons. You need to use the full equation which is
$E^2 = m^2c^4 + p^2 c^2$

3. Nov 10, 2015

### Phys_Boi

So what is the momentum of photons since gravitation is applied?

4. Nov 10, 2015

### Khashishi

Set m=0 and you get $E = pc$. You can solve for the momentum of the photon.

5. Nov 10, 2015

### Phys_Boi

Ah. Thank you

6. Nov 10, 2015

### ZapperZ

Staff Emeritus
Please note that this has been covered and answered in one of our FAQ.

Zz.

Last edited: Nov 10, 2015