1. General relativity and its verification Gravity bends light.
Why?

2. Gravity bends light. Why? First answer
“As Einstein developed his equivalence principle, he realized that it had some surprising ramifications. For example, his chamber thought experiment indicated that gravity would bend light. Imagine that the chamber is being accelerated upward. A light beam comes in through a pinhole on one wall. By the time it reaches the opposite wall, the light is a little closer to the floor because the chamber has shot upward. And if you could plot the beam's trajectory across the chamber, it would be curved because of the upward acceleration. The equivalence principle says that this effect should be the same whether the chamber is accelerating upward or is resting still in a gravitational field. In other words, light should bend when passing through a gravitational field. “ Walter Isaacson , “Scientific American”, September 2015

3. Gravity bends light as acceleration bends light

4. Gravity bends light. Why? Second answer.
«A beam of light carries energy and energy has mass. But every inertial mass is attracted by the gravitational field, as inertial and gravitational masses are equivalent. A beam of light will bend in a gravitational field exactly as a body would if thrown horizontally with a velocity equal to that of light. If the inside observer had reasoned correctly and had taken into account the bending of light rays in a gravitational field, then his results would have been exactly the same as those of an outside observer.”
Albert Einstein , “Evolution of phyisics” , from page 230 to page 232

5. Gravity bends light because light has energy and energy is mass
Photons don’t have mass but they have energy

6. Gravity bends light

7. Gravity bends light

8. Einstein was Right! Gravity Bends Light - The General Theory of Relativity from minute 3:00