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 CE–1039 CE) 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 does not. And with that single observation, a centuries-old theory vanished—in the blink of an eye.

SCIENCE SOURCE/COLOURIZATION BY:MARY MARTINScientists