Radiometer

The Radiometer is a large glass bulb or sphere with a mast running up the middle. Four diamond-shaped "sails" balance near the top. The object of the Radiometer is to convert light energy into mechanical and heat energy. Just put the Radiometer in the sun or near another strong light source. The speed that the Radiometer turns is dependent on the strength of the light. As the sunlight hits these flags, they begin to turn. In bright sunlight, they really spin fast! It's an amazing experiment in solar science and the nature of light. Recommended for children ages 8 and up.

What's included?

• Radiometer
  • 2.75" (7 cm) plastic base
  • 3" (8 cm) glass bulb
  • 5" (13 cm) tall
• Instruction and information guide

How does it work?

The glass bulb of the radiometer is sealed so that a tiny bit of air is trapped inside. This partial vacuum allows the flags to turn freely, without too much drag. Notice that the flags have one shiny side, and one black side. When the light hits the flags, it is reflected off the shiny sides, but absorbed by the black sides, which raises their temperature. When the black sides of flags heat up, the air near them also heats and the excited air molecules exert a tiny bit of pressure on the flags from one side. This small difference in pressure is enough to make the sails spin!

What does it teach?

Discover the relationship behind light, heat, and air pressure. Learn about the particle nature of light. When the photons strike the surface of the radiometer, they transfer their energy in the form of heat, and the sails spin.