4D20.10 Crooke's Radiometer

PURPOSE: To show radiation energy 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.

A PSSC filmloop called "The Pressure of Light" shows some experiments such as shooting a bb at a suspended metal bar to show that a rebounding particle imparts more impulse than a particle that sticks (they used grease to catch the bb). They also show a Crooke's Radiaometer.  It turned the "wrong" way. By refining the experiment with an ultra high vacuum pump and hanging the vane from a quartz fiber they were able to show the correct effect of  light pressure.

EQUIPMENT: Crooke's radiometer, lamp. 

SETUP NOTES: none.

edited by JZ 5/14/01