Double radiometer, after Crookes

The radiometer after Crookes is a light mill with four paddles on a vertical rotary shaft, within a glass bulb with low air pressure. The aluminium paddles are black at one side and reflecting at the other side. When sunlight falls on the light mill, the paddles will rotate. In 1873 Crookes thought to have proved the radiation pressure of the sun with his mill. But this interpretation is not correct, because the reflecting side would have to move away from the sun if the impulse of the light drove the paddles. However, the opposite is true. The most widely heard explanation is that the black side absorbs energy, which causes the molecules to vibrate faster here than at the shiny side. Therefore, the recoil of the air molecules colliding here is larger.