You can sit in physics class and learn a lot of things about light. Ask Stephen Hawking, who holds the Newton chair at Cambridge. He will tell you that light is the ultimate constant in the universe, that it always travels at 186,282 miles per second, that light transmits energy, radiation, and information. Or ask a third-grader to put a sunbeam through a prism and you will see the spectrum of a rainbow. Physics can tell us a great deal about light. But there’s one thing physics has never explained, namely, what exactly do we mean by that word “light”? What is it? We know it when we see it, but we can’t really explain what it is. Unlike space or time, light cannot be defined over against anything else. Light simply exists. What does it mean for Jesus to say, “I am the light of the world”?