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Crooke's Radiometer

Crooke's radiometer consists of a vane mounted on a pivot enclosed in a glass bulb filled with air at low pressure. One side of each vane is painted black and the other side is painted silver. The black side absorbs more radiation than the silver side so heats up more. Air molecules hitting the black side gain more kinetic energy that air molecules hitting the silver side, rebounding with a higher velocity, causing the vane to rotate.

If the air pressure is reduced to closer to a vacuum, the vane rotates in the opposite direction, because of the radiation pressure on the shiny surfaces. Radiation is reflected from the shiny surface and absorbed by the black surface.

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