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. Two 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. (2) Photons are mostly absorbed by the black side, but mostly reflected by the white side, thus heating the black side more. Air molecules coming into contact with the vanes will obtain more heat energy from the black sides, colliding with greater momentum transfer from that side, thus producing rotation away from the black side. This is the correct result and explanation.

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.


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

EQUIPMENT: Crookes' radiometer with matches.


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