35. Light fan

Hold a light-coloured rod between your thumb and forefinger and move it quickly up and down in neon light. You do not see, as you might expect, a blurred, bright surface, but a fan with light and dark ribs.

Neon tubes contain a gas, which flashes on and off 50 times a second because of short breaks in alternating current. The moving rod is thrown alternatively into light and darkness in rapid sequence, so that it seems to move by jerks in a semicircle. Normally the eye is too slow to notice these breaks in illumination clearly. In an electric light bulb the metal filament goes on glowing during the short breaks in current.