The radiometer shown here has four vanes painted white on one side and black on the other. They attach symmetrically to a glass thimble which balances on a sharp needle. The glass container has been sealed under partial vacuum. In my office the radiometer sits with the vanes stationary but they spin rapidly in the flood lamp. Why?

We know light is absorbed and reflected. The quantum theory of light says that each photon carries momentum. We know that a floodlight is warm. We can think of light as a ray or a wave or quantum mechanically as some strange mixture of the two.

I suggest two possible explanations:

(1) The dark side of the vane absorbs radiation and heats up relative to the white side. Thus the few gas molecules left in the bulb, when contacting the surface, rebound from the dark side with more energy than from the light side due to the radiation heating.

(2) Photons carry momentum and thus exert a force on the vanes. Estimate this force for the situation shown above, where a 75 Watt flood lamp illuminates the vanes from a distance of about 10 cm.

Which of the two possibilities discussed above do you favor and why?

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