Crooke’s Radiometer

You’ve seen these in the gift shop of the science museum.  Inside an evacuated bulb is a 4-vane windmill, each vane black on one side and white on the other.  Shine light on it, and it spins.

Image result for crookes radiometer gif

Imagine a tiny pea-shooter aimed at the vane.  If the pea sticks to the wall, it gives all its momentum to the wall, pushing it forward.  But if the pea bounces off the wall, it gives twice its momentum, because the momentum of the pea is reversed on the way out.  Photons of light have only a tiny momentum, but the principle is the same.  The light that is absorbed on the black vanes gives its momentum to the vane, while the light reflected from the white vanes imparts a double push, because the light coming in and the light leaving have equal and opposite momentum.

This is the explanation for the motion of the vane that I imagined.  It was proposed by Sir William Crooke when he sent in the results of his experiment to the science journal of the Royal Society in 1973.  The person assigned to review the submission was none other than James Clerk Maxwell, who had explained the electromagnetic nature of light with the 4 equations that are now associated with his name.  Maxwell noticed that the vane spins in the wrong direction, as though the push on the black side were stronger than on the white side.

The real explanation, and the rest of the story

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