HS7610 Crookes’ radiometer

The radiometer’s whirl starts to spin when exposed to a light source; the greater the intensity of the radiation, is the quicker it spins around. This fact is due to the gas particles inside the radiometer: when they hit the black sides of the whirl’s paddles, which are warmer than the white sides because of a greater light-absorbent power, the particles bounce quicker and, therefore, give an impulse which is greater than the one given to the white sides. Thus the whirl’s rotation is generated.