Have you seen a light bulb shaped, paper thin glass envelope that contains what looks like a weather vane with four diamond shaped metal leaves inside? Black on one side and white or silvered on the other side? When someone takes it from a shelf and puts it in the window, it spins in the sun. It is named for Sir William Crookes who first devised it in 1873. It is sometimes called a light mill.
Crookes radiometer appears to be a cross between a scientific invention and a child’s toy. The atmosphere inside the bulb is a modest rather than a complete vacuum, so resistance to vane rotation is minimal. It rotates as if the sun somehow pushes the black surfaces.
Crookes Radiometer – Why a Big Deal?The Crookes radiometer wouldn’t be such a big deal, but no one, not even the scientists, knows exactly what makes it work. Clearly, it has something to do with the sun and the color contrast of the metal surfaces. Then there are the gas molecules. When the bulb is evacuated to a high vacuum, the vane no longer rotates.
Below, we present some of the theories that attempt to explain what makes Crookes radiometer go round. Some of them have been disproved. The others remain viable possibilities. Perhaps you have a suggestion you’d care to offer?
Explanation 1: Collision MomentumThe obvious explanation is that light particles are absorbed by the blackened surfaces, imparting momentum that results in rotation. The catch is, light that bounces off the other surfaces imparts even more momentum, so the vane should rotate in the opposite direction! Explanation 1 is incorrect.
Explanation 2: Pressure from Heated GasThe next theory sounds a little more convincing than the first theory. Photons notably of the infrared region of the spectrum absorbed by dark surfaces provide thermal energy that heats adjacent gas molecules. This pressurizes gas molecules at the surface, which shoves the vane in the opposite direction. Part of the difficulty with this theory is that the heat radiates in all directions, in a weak, steady flow. The answer appears to lie elsewhere.
Other “False” TheoriesOther theories that don’t explain the phenomenon of the Crookes radiometer include the out-gassing effect and the photoelectric effect. It ought to be mentioned a single action may be the net result of multiple causative factors. So while there is clear evidence the above-discussed mechanisms don’t explain the rotation in a Crookes radiometer, they doubtless play some small role in the balance of forces. There is probably a combination of factors involved.
Probable CauseThe primary factor may be the “thermal transpiration” mechanism. In this mechanism, the hotter gas molecules near the dark surfaces of the vane travels around the edges of those surfaces to the other, cooler sides, imparting some force to the edges. Cooler molecules traveling in the reverse direction possess less energy and so the force they impart in the opposite direction is considerably less. The net result is vane rotation in the expected direction.
We did say “probable cause” was the theory of “thermal transpiration,” but in truth, the debate rages on.
Afterword: Reflecting on the above, the author wishes he was in a position to explore 1) the effect of changing gas composition to a pure gas, 2) a gas of greater and a gas of lesser atomic or molecular weight, and the effect produced by limiting the light to a single frequency.
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