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File:PSM V09 D291 Effects of temperature on light.jpg

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English: Two radiometers built by British physicist William Crookes to try to demonstrate the mechanical pressure exerted by light. From William Crookes (1876) "The Mechanical Action of Light", Popular Science Monthly.

In the device on the left, a card that is reflective (white) on one side and absorbent (black) on the other is suspended from a fine glass torsion fiber in a vacuum. Light striking the white side is reflected back, imparting twice the force to the card as light striking the other side which is absorbed. So the greater force on the right side was supposed to cause card to turn slightly, twisting the fiber.
The device on the right is called Crookes radiometer. It consists of a light turbine suspended on a sharp pointed pivot, consisting of vanes that are black on one side and white on the other. Again light striking the white vanes was supposed to exert more force than the light striking the black, causing the turbine to rotate.

Crookes found that both these devices turned, which he believed was due to the mechanical force of light. However it was later shown that the movement was not due to the force of the light, which was too weak to be detected, but instead due to the radiometric effect. When the light hit the surfaces it was absorbed, raising the temperature. More light was absorbed by the black side, so the black surface got warmer than the white. The air next to the black surfaces was heated and expanded, pushing the surfaces away from the black side.
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Source Popular Science Monthly Volume 9
Author Unknown authorUnknown author
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