The theory of relativity states that gravity does effect light but how and why? Light does not have mass? Photons are energy not exactly atoms. They are not known to have any such weight/mass.
In Relativity, gravity is not the attraction between masses. It is the curvature of spacetime due to all kinds of energy, momentum, and their flux. All bodies, including photons, then follow that curvature.
Light has energy and Einstein established the link between energy and mass. It is this link that allows gravity to affect light.
