Radiometer
Radiometer, an instrument designed to detect and measure radiant energy. A simple type, devised by Sir William Crookes, consists of four vanes of mica or aluminum foil that revolve on a needle point within a glass globe from which most of the air has been removed. One face of each vane is blackened, while the other is bright.
When placed in sunlight or near any light, the vanes rotate in the direction faced by the bright sides. This rotation is due to the fact that a black surface absorbs and radiates heat while a bright surface reflects heat. The greater heating of the blackened surfaces, according to one theory, causes nearby air molecules to move more rapidly and strike more blows on the darkened faces.
The Crookes radiometer is used mainly as a classroom demonstration device and as a toy. More complicated radiometers are used to make scientific measurements of radiant energy.