Question : Why does the mass of a body change when it emit or absorb photons ?
Ans. As photon has energy hν and according to theory of relativity, energy E is equivalent to a mass (E/c²), so photon has a mass (hν/c²). So theoretically the mass of a body will decrease if it emits photons and will increase if it absorbs them. However due to small value of h =(6.6 x 10-34 Js) and large value of c( = 3 × 108 m./s) as the mass of an optical photon is too small, the change in mass of a body due to emission or absorption of photons is practically negligible as compared to the mass of the body.