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Crookes Radiometer

Infinity candle

Crookes radiometer is a fascinating physics gadget and decorative item that converts light thermal energy into kinetic energy...

The Crookes radiometer consists of four vanes mounted on a spindle in a partial vacuum inside a glass bulb. When the vanes are illuminated, they start to rotate. In bright sunlight, they can spin several thousand revolutions per minute. Theories about why the blades start to move have been put forward for over a century. Initially, it was assumed that the driving force is the pressure of light, but experiments showed that this is not the correct interpretation. The currently accepted theory was proposed by Osborne Reynolds, and it explains the motion of the blades by the phenomenon called thermal transpiration. Since one side of the vane is darker than the other, it heats up more under the influence of light. Warm air naturally begins to move from the warmer side of the vane to the cooler side, and this movement of thin gas causes the rotor to spin.


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