Author(s): Craig Didden, Rosie Van Zyl
Demonstration Equipment - Teacher's Guide
SED 695B; Fall 2005

Principles illustrated:•

To observe the movement of the vanes in a radiometer due molecular movement caused by different light types.

Physics:
4. Waves have characteristic properties that do not depend on the type of wave. As a basis for understanding this concept:

a. Students know waves carry energy from one place to another.

Materials:

• Sunny Day
• Fluorescent Light
• Incandescent Light
• UV Light
A radiometer consists of a set of vanes, each white, or light in color, on one side and blackened on the other. When exposed to light, the vanes revolve. The black surface is warmer than the white one and that gas molecules will recoil faster from the hot surface. The difference in molecule movement is what causes the device to spin. The first radiometer was constructed to settle the controversy regarding whether light exerts a force. Even though it failed to do this it did show that the dark surface would cause the air molecules to recoil faster from the dark surface than the light surface. When different types of light are used the vanes of the radiometer will spin at different speeds. This is due to the energy from the light source. Each different light source will give off different wave lengths due to the type of light bulb being used.

Procedure:

1. Place radiometer in full sunlight, make sure that there is no cloud cover, or that it is minimal.
2. Make some observations about the rotation of vanes while it is outside. Talk about the speed of the vanes
3. Place the radiometer under a fluorescent lamp in the lab. Make sure to turn off the overhead lights in the classroom. Observe rotation of vanes and make some observations about the speed of the vanes.
4. Place radiometer under an ultraviolet lamp in the lab. Make sure to turn off the overhead lights in the classroom. Make some observations about the speed of rotation of the vanes.

5. Place the radiometer under an incandescent light source. Make some observations about the speed of the vanes in the radiometer.

Students observing rotation of the radiometer, outside on a sunny day. It is important that there is little to no cload cover on the day of observations
Students observing rotation of the vanes of a radiometer inside a classroom using a fluorscent light source.
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