Physics Puzzle: Crooke’s Radiometer

Posted on April 15, 2008
Filed Under Physics Puzzles |

Here’s a riddle about a well known object known as Crooke’s radiometer. This is a glass “bulb”-like object with four plates fitted on a metal rod, free to rotate (much like a horizontal windmill). Each plate has a black side and a silver side, as is evident from the photo:

Crookes radiometer

The inside of the bulb is kept at a perfect vacuum. Here’s the riddle: what will happen when the radiometer is placed in direct sunlight?

Scientist X argues that the plates will rotate clockwise, i.e., the silver side of the plates will face the positive rotation angle, because the black side will absorb photons, and hence momentum from light, which will “push it forwards”, like blowing wind on it.

Is scientist X correct?

What will happen as air is slowly (slowly!) let into the vacuum?

Good luck & enjoy!

Comments

2 Responses to “Physics Puzzle: Crooke’s Radiometer”

  1. admin on April 18th, 2008 5:44 pm

    HINT (ADDED 19/4/2008):

    You can order radiometers from science toy stores. Buy one and try it out! Which way does it turn? Is it similar to the situation described in this question? (Hint: “our” radiometer is kept at a perfect vacuum. What mechanisms can turn the plates other than light pressure? Can you estimate the pressure, by the way?)

  2. admin on April 25th, 2008 3:49 pm

    ANOTHER HINT (ADDED 25/4/2008):

    When kept in perfect vacuum, the ONLY interaction of the radiometer’s plates can be via radiation.

    When not in perfect vacuum, you will need to be able to estimate which effect is greater: the photon pressure, or the molecule pressure. Can you estimate for what density of air molecules do the two effects become equal in magnitude?

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