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Radiometer in Full Vacuum
Name: Garrick
Status: student
Age: N/A
Location: N/A
Country: N/A
Date: N/A
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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Update: June 2012
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