# Why photons have momentum?

1. Mar 12, 2012

### suchal

p=mv then for a photon p=0*c means p=0. And also ke=1/2pv which is ke=0 and E=mc2 means E=0. please use easy words and we descriptive.

2. Mar 12, 2012

### dextercioby

For <particles> of light, photons, certain care needs to be taken about what p,m,v,E mean. If p=mc, then m is the so-called 'relativistic mass' which has no meaning for a photon, but only for a massive particle.

The universal formula which is to be used is

$$E^2 = \vec{p}^2 c^2 + m^2 c^4$$

where m is the rest/invariant mass which is 0 for a photon.

3. Mar 12, 2012

### lugita15

See the FAQ here. Also, you might find this thread interesting.