Answer 1

Relativistic equation of energy :

`E^2 = p^2c^2 + m^2c^4`       ....(1)

Here, p²c² = kinetic energy of photon
           m₀²c⁴ = internal energy of photon
We know photons have zero rest mass. Therefore, m₀ = 0. Substituting the value of m₀ = 0 in equation (1), we get : `E = pc`

Thus, the energy of a photon should be called its kinetic energy.