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Q & A: rotating vane radiometer

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Most recent answer: 11/15/2009
Q:
My dad told me about this instrument but could not remember the name. It has vanes (white on one side and black on the other side)set on a spindle and inside a vacuum tube. The vanes turn when exposed to light. What is the name of the instrumen? Do the vanes turn because the light photons hit the vanes or because the light has mass, or because the white vanes repell the light? Can you suggest a place I can purchase this instrument? My science project is how light photons create loose electrons to become electrical flow, and I think this instrument would be a great start to my science presentation. Mario Presutti
- mario presutti (age 10)
oviedo, florida
A:
 It's called a radiometer: .

The way it works is unfortunately not very elegant. The light heats up the black sides more than the white sides, since black absorbs the light energy. That in turn heats the gas in the container more on the black side, increasing the pressure on that side, thus pushing the vanes around.

If they were driven by the light  momentum (which is very small) they would turn the other way. That's because light bouncing off  a white side transfers twice as much momentum as light absorbed on a black side.

The photoelectric effect (knocking electrons loose) isn't part of this story. However, the device does illustrate the absorption of light energy.

Mike W.

(published on 11/15/2009)

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