a
(a) According to Einstein's quantum theory light propagates in the form of packets i.e. quanta of energy, which is called photon.

The rest mass of photons is being zero. It can be shown, according to the relativity theory of light.

According to the relativistic theory equation, the mass of the photon is computed as:

\(m=\frac{m_{0}}{\sqrt{1-\frac{v^{2}}{c^{2}}}}\)

\(m_{0}=m \sqrt{1-\frac{v^{2}}{c^{2}}}\)

when \(v=0\)

So \(m_{0}=0\)

Where \(m_{0}\) is the rest mass of the Photon.