PURPOSE: To stimulate discussion about radiative heat transfer and conservation of momentum with photons.

DESCRIPTION: A match or other source of light is brought near the radiometer, resulting in rotation of the vanes. Possible explanations for this include: (1) Photons are mostly absorbed by the black side of each vane, but are mostly reflected by the white side, thus transferring more momentum to the white side, producing rotation in the direction of the black side with the white side trailing; and (2) The black side of each vane becomes warmer due to absorption of heat from the source, and therefore molecules of gas interacting with the black side obtain higher kinetic energy. When they leave the black side they therefore introduce a greater reaction force on the black side, producing rotation in the direction of the white side with the black side trailing. Neither of these possible explanations are correct: (1) because the rotation is in fact with the black side trailing, and (2) because this explanation is fundamentally flawed.

The REAL reason has to do in a very important way with details regarding how molecules interact with each other. Much of the original research work into this device was carried out by Crookes and Reynolds. I will quote from an excellent! article in The Physics Teacher magazine: Arthur E. Woodruff, The Radiometer and How it Does Not Work, TPT 6, 358-363, (1968):

...Imagine two parallel plates of indefinite extent, arranged as in a parallel plate capacitor and connected to each other, with a gas in the space between. If we naively accept the argument Reynolds originally gave, then, if one of the plates is warmer on its inner surface than the other, the entire apparatus should accelerate with the warmer plate leading, without any external forces. This, of course, contradicts the fundamental principles of mechanics......

. . . though the molecules make more vigorous collisions with the warmer side of the vane, they are held back more effectively from the vane by the recoiling molecules, so that the pressure over most of the vane is the same as the pressure on the cooler side. But for molecules impinging near the edge of the vane, onto a strip of order of the mean free path in from the edge, the situation differs. They are held back in part by the molecules rebounding from the vane from the cooler side. But the latter are less efficient in stopping incoming molecules. So while the individual collisions with the vane may be on the average just as vigorous as nearer the center of the hot surface, more such collisions will occur in a unit area in a given time. The pressure near the edge, then, will be greater than at the center of the warm side, and therefore also greater than the pressure on the other side. It is this excess pressure at the edge which is responsible, at least in part, for the motion of the vanes. . . .

The explanation is not nearly as simple as the difference in the momentum of photons when they are absorbed or reflected, or even as simple as the heating effect on the black side, which absorbs more photons, compared with the white side, which reflects more. This appears to be one of those physics devices that is typically explained incorrectly, even in the literature from the supplier that accompanies the radiometer.

In the movie "Pressure of Light" it is shown that reason (1) could not apply to this device even if the vacuum were very good.

SUGGESTIONS: See Question of the Week #173 and #174 for suggestions on using this demonstration to enhance class involvement.

REFERENCES: (PIRA 4D20.10) See Demonstration Reference File for information and articles on this device.

EQUIPMENT: Crookes' radiometer with matches.

SETUP TIME: None.