Key Takeaways
- A Crookes' radiometer consists of an airtight glass bulb with a partial vacuum and a set of vanes that spin when light is shone on them.
- The spinning of the vanes is attributed to thermal transpiration, where light heats the black side of the vanes, causing the air on that side to be expelled faster and the vane to move away from the light source.
- The effectiveness of the radiometer depends on a good but incomplete vacuum; in a full vacuum or without vacuum, the vanes do not spin, due to lack of air drag or the inability of thermal transpiration to occur.
A Crookes' radiometer has four vanes suspended inside a glass bulb. 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 rotations per minute!
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, causing them to rotate. However, this force is exceedingly small.
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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.
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