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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.
