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Name: Garrick
Status: student
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Question:
If you had a radiometer with a complete vacuum inside, would it move with the black vanes leading from the force of light photons rebounding off the white vanes?



Replies:
Theoretically, it should, but you would need a very intense light source, and the radiometer would get very hot. I remember when I was in high school seeing a movie on that phenomenon. To actually move a reflective piece of metal with light required much more stringent conditions than a radiometer would provide. A small piece of foil was suspended from a long quartz fiber in a highly-evacuated tube, and illuminated on one corner with a focused auto headlight. The foil just barely turned. In the same movie, a highly- evacuated radiometer simply remained motionless when illuminated with ordinary sunlight.

Richard Barrans, Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming


Yes.
A photon absorbed imparts its whole momentum into the absorbing surface, because its mass goes from moving to stopped. But a photon reflected imparts up to twice its whole momentum, because it not only stops, it re-starts moving back the way it came. The white side definitely gets more net pressure or thrust than the dark side. How much more, that may be influenced by the direction-scattering effects of being flat white color instead of mirror-like silver color.

Jim Swenson



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