Crookes' Radiometer

Purpose

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

Equipment

Crookes' radiometer with matches.

 

Images

i2-01

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 
 

References

Gordon F. Hull, Concerning the Action of the Crookes Radiometer, AJP 16, 185-186, (1948). H. V. Neher, Light-Pressure Tube, AJP 29, 666-668 (1961)