Equipment:

• Radiometer
• Light source with power supply

Demo:

Four vanes (one side white, one black) are suspended on a pin. When light is shined on the apparatus, vanes rotate in the direction of their white sides.

Explanation:

The vanes move because the black surfaces are hotter than the white ones, so they give off more energy to the air molecules. When a molecule touches the hot side it bounces off with more energy due to having more heat energy.This causes it to spin in the direction of the white sides. The more intense the light source the faster the vanes spins.