Radiometer and Light Bulb

The vanes of the radiometer are painted black on one side and silver on the other. The vanes are inside a (mostly) evacuated tube. When placed near a light source, the vanes rotate.

The tricky part here is that the radiometer rotates the opposite direction from what one would expect. You cannot explain the motion with conservation of momentum of the photons. You have to invoke the effects of gas in the radiometer. Play with this one before demonstrating in front of a class.