Light

In 1696, Britain taxed houses by window count. Homeowners bricked up windows to cut the bill, leaving rooms without enough light or ventilation. Doctors opposed the health effects; the tax was repealed in 1851. Bricked-up window outlines remain on old buildings.

The Human body emits Visible light. A 2009 Tohoku University study used an ultra-sensitive camera to show the face, neck, and chest glowing faintly. This is not infrared heat but biophotons from metabolic reactions, about 1,000 times too dim for the naked eye.

That a rainbow has seven colors — 'ROYGBIV' — was Isaac Newton's choice. He first saw five in the prism's spectrum, but believing colors should number seven like a scale, he added orange and indigo. Indigo is the most disputed band today.

A photon born in the core of the Sun takes tens of thousands to hundreds of thousands of years to reach the surface. Trapped in ultradense plasma, it is absorbed and re-emitted in a random walk. Once it escapes, it crosses 150 million km to Earth in just eight minutes.

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.