Classical thinkers like Euclid and Ptolemy believed that our eyes work by emitting rays that travel to the objects we see. Ibn al-Haytham [965–1039] reasoned that if this were true, then when we open our eyes it should take longer to see something far away than something nearby. And guess what? It doesn’t. And with that single observation, a centuries-old theory vanished—in the blink of an eye.

SCIENCE SOURCE/COLORIZATION BY: MARY MARTIN