A white/black fan placed in a sealed flask spins when exposed to light!


Teachable Topics:

  • light pressure
  • quantum nature of light
  • radiation


When light strikes a surface, some of the light is absorbed and transformed into heat, while some of it is reflected. A black surface absorbs more than it reflects, while a metallic surface mostly reflects the light. Consequently, the black surface becomes hotter then a metallic one if both are exposed to the same light.

The radiometer has four panels mounted on a pin-tip so that it can spin freely. One side of each panel is painted black, while the other side is bare metal.

Most of the air has been removed from the bulb which contains the four panels. The panels can thus spin quite freely. The air that remains, though, comes in contact with the surfaces of the panels. The air that touches the black side gets heated more than the air that touches the metal surface. The hotter air has a greater pressure, so the push of the on the black side of the panel is stronger than that on the metallic side. As a consequence, the array of panels start to spin.

The more intense the radiation, the more energy is transferred to the panels, and the faster the array of panels spins.

Figure 1: Diagram of Radiometer Pane

Figure 2: Photo of Radiometer 


  • Non-fluorescent lights
  • Radiometer apparatus


  • Place the Radiometer apparatus on a flat surface. When the radiometer is illuminated with non-fluorescent light, the black side of its panels absorb the light, heating the air and giving it a higher pressure. The bare metal reflects the light, so the metal heats/pressurizes the air less. You will see the four panels start to spin as the light shines on them.


  • This apparatus is very delicate, and should be handled with great care.
  • Though not shown on this page, it is possible to effectively eliminate the effects of heating the air by pouring a cryogenic liquid (e.g. liquid nitrogen) over the bulb. Without the pressure of the air, one can then observe the effects of radiation pressure, which will turn the panels in the opposite sense. But be very careful not to crack the bulb with the cold nitrogen!