I teach science to a first grade class. Could you please explain if
the movement of the blades in a radiometer is due to the light or the
heat produced by the light.
The movement is caused by gas molecules that are still in the
radiometer. (There is a good, but not high, vacuum in these objects.) When the
radiometer is illuminated, the black sides of the vanes absorb most of the
light falling on them, and the bright (white or shiny) sides reflect most
of it. Hence, the black sides become warmer than the bright sides. When a
gas molecule comes into contact with one of the vanes, it rebounds more
strongly from a black face than from a bright face (because it gains more
energy from a warmer face than a cooler face); as a result of many such
rebounds, the vanes rotate, with the bright faces leading.
It is true that light striking an object does give some momentum to that
object, but the amount is extremely tiny; furthermore, if the radiometer's
movement were due to this "light pressure", the vanes would spin with
the black faces, not the bright faces, leading.
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Update: June 2012