Transcript: One of the most important effects of the general theory of relativity is the fact that mass or gravity can bend light or electromagnetic radiation. To see this, consider again the example of a sealed elevator. If the elevator was in free space being accelerated and you stood in the elevator and shined a flashlight across the elevator, you can see that as light traveled across the elevator it would be deflected downwards by a tiny amount caused by the motion of the elevator itself. But since we cannot distinguish this situation from the elevator stationary on the surface of the Earth with the force caused by gravity, gravity must cause the same effect, and in fact it is the case. Mass will deflect light. The effect was measured in1919 during a total eclipse of the Sun by an eclipse expedition led by the astrophysicist Arthur Eddington. The observation of a tiny deflection of light around the limb of the Sun by the mass of the Sun, an effect of barely one arcsecond, was sufficient to confirm the general theory of relativity. It helped to make Einstein famous.