Yes, a beam of light is affected by gravity. This might seem counterintuitive, as light is massless. However, gravity affects the spacetime itself, and light travels along the paths determined by this curved spacetime.

Here's how it works:

* Spacetime Curvature: Massive objects, like stars and planets, warp the fabric of spacetime around them. Imagine a stretched sheet with a heavy ball placed on it. The sheet will curve downwards around the ball. Similarly, massive objects distort the spacetime around them.

* Light Follows the Curve: Light always travels in straight lines through spacetime. However, if spacetime is curved, the straight line path of light appears to bend from the perspective of an observer. This bending is what we call gravitational lensing.

* Einstein's General Relativity: Albert Einstein's theory of general relativity explains this phenomenon. It states that gravity is not a force, but a consequence of the curvature of spacetime caused by mass and energy.

Evidence:

* Gravitational Lensing: Astronomers have observed this effect in many cases, such as light from distant galaxies being bent around massive clusters of galaxies.

* Light Bending Near the Sun: During a solar eclipse in 1919, astronomers observed that starlight passing near the Sun was bent, confirming Einstein's predictions.

So, while light itself doesn't have mass, it is affected by gravity through its interaction with the curved spacetime created by massive objects.