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radiometer paradox



I am puzzled by how a radiometer works. It makes sense to me that
when you shine light on the black fin that radiation pressure would
cause the radiometer to spin. However, why doesn't it spin even
faster when you shine light on the white fin? In fact, it doesn't
spin at all! If I think of
reflection as a photon of light bouncing off of the white
fin, then the change in momentum of the photon would be twice as
great as if the photon were absorbed.

I looked up radiation pressure in several text books. The closest
example indicated that radiation pressure = the light irradiance
(watts per m^2) / speed of light; i.e. P=I/c if the light is
completely absorbed whereas P=2I/c if the light is reflected. This
matches my simple picture of what I thought was happening when light
hits the radiometer fins.

So, why doesn't light shining on the white fin (which is reflected) cause it to spin
faster than light shining on the black fin (which is absorbed).

Joseph Gress
**************************************

Sincerely,
Joseph Gress
Department of Physics and Astronomy
Moorhead State University
Moorhead, MN 56563
e-mail: gress@mhdha.moorhead.msus.edu