4D20.10 Crookes' Radiometer


Thermal vibrations in solids, mean free path of a gas, absorption of light, thermal transpiration


Four vanes are black on one side and white on the other. They can freely rotate about a pivot in a partial vacuum. When light shines on the vanes, they rotate due to additional force from gas collisions on the black surfaces. There are two significant contributions to this behaviour.

Firstly, the black surfaces absorb more energy from the light. The hotter black surfaces impart a larger impulse on the surrounding gas due to the stronger thermal vibrations. While higher-energy gas particles tend to block more gas particles from reaching the vanes, cancelling out the effect, this is not true on the edges of the vanes.

Secondly, there is an additional effect that further contributes to the rotation: thermal transpiration or thermal creep. Cold gas from the white sides shifts over to the hot, black sides, increasing the pressure on the black sides and giving a further push.

The relative importance of these two mechanisms requires detailed discussion, found in the references and the technicals section.



  • [1] Crookes' radiometer
  • [1] Incandescent flashlight


  1. Shine the flashlight at all vanes from close range. The black sides should move away from the light.


Additional Resources



  • Don't attempt this at home!

Last revised

  • 2019


  • When the mean free path of the gas particles is on the order of the radiometer size or larger, the collision mechanism is important. As the mean free path shrinks, particles bouncing off the black surfaces are more likely to intercept other particles moving towards the black sides, mitigating some of the effect of the push. At all pressures, the collision mechanism remains important on the edges of the vanes.
  • When illuminated with a smartphone flash or red laser pointer, the radiometer does not rotate. When illuminated on a vane edge by a green laser pointer, it rotates, but not if illuminated in the middle. It would be interesting to try the demo with a variety of light sources of various powers to see what works.

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If you have any questions about the demos or notes you would like to add to this page, contact Ricky Chu at ricky_chu AT sfu DOT ca.