We talked about how electromagnetic radiation or light displays characteristics of both waves and particles. Another question we were wondering about was whether or not these particles, photons, have mass. Since light is affected by a gravitational field, does that mean they have mass?
The answer is no, photons are believed to have no mass. To be affected by gravity, they would not need to have mass. Rather, light travels in a straight line through spacetime, which is distorted near a massive object like a planet, star, black hole, etc. Spacetime is a unified continuum taking into account both space and time. The concept of spacetime comes from the special theory of relativity. Special relativity was developed by Einstein, and famously also posits the equivalency of mass and energy, which everyone recognizes as E = mc2.
Light traveling in a straight line through spacetime around a star curves, making the source of the light appear to be in a different place. This has been experimentally verified by observing stars around the sun during eclipses. The above graphic is taken from this website, which is throwing its own Clear Science on the question (check it out).