Light

In 1818, Siméon Denis Poisson argued that if light were a wave, a bright spot should appear at the center of a circular object's shadow — which he thought was absurd. But when Arago actually ran the experiment, the bright spot appeared, proving light's wave nature.

The "photic sneeze reflex," nicknamed ACHOO syndrome, causes people to sneeze when suddenly exposed to bright light. It's surprisingly common—affecting 18–35% of Americans and about 25% of the Japanese population.

Purple does not exist in the light spectrum. When red and blue light simultaneously stimulate the eye, the brain creates this color to resolve the contradictory signals.