Question

Is light affect by gravity. why? ​

Answer 1

Since light has no mass, how can it be trapped by the gravitational pull of a black hole? Newton thought that only objects with mass could produce a gravitational force on each other. According to Newton's theory, the force of gravity should not affect light

Answer 2

This is a wonderful question … in the first instance, it’s answered by Einstein’s theory of general relativity. (Spoiler alert, light travels along geodesics, which are approximately straight lines in most circumstances that humans encounter in everyday life, but are curved by gravity). It’s also interesting because there is some sense in which light does have “mass” (and therefore should be influenced by gravity… read on!)

In our everyday experience, light seems to travel in straight lines, unaffected by gravity. Of course, light can bend when it passes through the interface between two media — think of light refracting as is passes from air into water, which is the phenomenon that causes a straw in a glass of water to appear kinked at the interface. But that bending is not gravitational; it’s electromagnetic.

However, light does bend when travelling around massive bodies like neutron stars and black holes. This is explained by Einstein’s theory of general relativity.

We are all familiar with massive objects being influenced by gravity. For instance, think of a planet orbiting the sun. As the planet moves, a centripetal force acts on it, which curves the motion. Without gravity, the planet would travel in a straight line. General relativity puts a different perspective on the situation. Instead of describing the object as moving along a curve in a flat spacetime, the object is described as moving along special “lines” in a curved spacetime. The curves are a consequence of gravitation. The spacetime curves are called geodesics, and they generalize the notion of straight lines to curved spacetime.

Light also travels along geodesics (called null geodesics), and so paths of light are also curved by gravitational force, despite the light not having any mass.

There is some more cutting-edge research related to this answer. There is reason to believe that light itself curves spacetime in the same way that massive objects do. This is sometimes referred to as the self-gravitation of light. The idea is that an electromagnetic wave has a non-zero energy-momentum tensor, and should therefore curve spacetime, albeit in a small and strange way. In this way, the equations of general relativity imply that the spacetime curvature created by propagating light should influence the propagation of that light itself.