Originally Posted by **binayakafle** Prove that dynamic mass of photon is hv/(c^2)
where notations carry usual meaning. |

Physicsquests derivation is correct but it requires one to know that E = mc^2. Here is a derivation which does not require the reader to know that. All the reader is required to know is the classical expression E = pc which can be derived for an EM wave.

The actual name of such a mass is

**inertial mass** or as others like to call it,

**relativistic mass**. The mass that people refer to when they say that the mass of a photon is zero is known as

**proper mass** or as

**rest mass**.

The derivation is quite simple. By

*definition*, the inertial mas of a particle is

the

*m* in the relation

**p** =

*m***v** or if we're speaking only of magnitudes,

*p* =

*mv*. For a photon

*v = c* and

*E = pc* ->

*p = E/c*. So we have

*E/c = mc*. Solve for

*m* to obtain

*m = E/c^2*. The energy of a photon is related to the photon's frequency

*f* by

*E = hf* so we have

*m = hf/c^2.*