How does a crookes's radiometer work?

★ wanayoo — archive 1999 http://www.howstuffworks.com/question239.htm

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Question

I once saw device shaped like a light bulb. It had a vertical support inside it, and on that support there were four vanes with four diamonds on the end. One side of the diamond was black and the other was white. I did a little research and found out that it was called a Crookes's Radiometer, but I can't figure out how it works. How does a Crookes's Radiometer work?

Answer

A crookes's radiometer has 4 vanes suspended inside a glass bulb as you've described (see this nice picture). Inside the bulb there is a good vacuum. When you shine a light on the vanes in the radiometer, they spin - in bright sunlight they can spin at several thousand RPMs!

The vacuum is important to the radiometer's success. If there is no vacuum (that is, if the bulb is full of air), the vanes do not spin because there is too much drag. If there is a near-perfect vacuum, the vanes do not spin unless they are held in a frictionless way. If the vanes have a frictionless support and the vacuum is complete, then photons bouncing off the silver side of the vanes push the vanes and they rotate. However, this force is exceedingly small.

If there is a good but incomplete vacuum, then a different effect called thermal transpiration occurs along the edges of the vanes, as described on this page. The effect looks as though the light is pushing against the black faces. The black side of the vane moves away from the light.

Here is the definitive link on how radiometers work:

How does a light-mill work?

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