Photons and gravity: It is observed that light is affected by gravity. We can recognize the gravitational behaviour of light from the fact that although a photon has zero rest mass, it interacts with electrons as though it has inertial mass.

Gravitational Red shift: As we know that an interesting astronomical effect because of the gravitational behaviour of light i.e., the frequency associated with a photon moving towards earth enhances and the frequency of a photon moving away from earth should reduce.