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Radiometer Lab Supplement

Radiometer Lab Supplement

Author: Kyle Joslin

This assignment is designed to teach you information about Radiometers as a replacement for performing the actual lab. 

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Watch this video with HEADPHONES! If you do not have headphones, turn on the captions at the bottom and turn the sound off.

How do Radiometers work?

The Radiometer provides an introduction to the study of thermodynamics that sits on your desk or table top. Set it in sunlight or incandescent light and watch the mill inside spin!

The radiometer consists of four vanes, each has one blackened side and one silvered side. It is encased in a clear glass bulb that has a near perfect vacuum. So what happens to make the radiometer "work"? 

Crookes originally believed that sunlight falling on the light-mill turned the vanes so that the black surfaces were being pushed away by the light. He thought that this demonstrated that light radiation pressure on the black vanes was turning it round just like water in a water mill.

The problem with Crookes' explanation is that light falling on the black side should be absorbed, while light falling on the silver side of the vanes should be reflected. This means that there would be twice as much radiation pressure on the metal side as on the black and the mill would actually turn in the opposite way.

What really happens is that the faster molecules from the warmer side of each vane strike the edges obliquely and impart a higher force than the colder molecules. The movement of the vane as a result of tangential forces around the edges, is away from the warmer gas and towards the cooler gas with the gas passing round the edge in the opposite direction. Therefore, the movement is actually due to what happens near the edges of the vanes rather than at the faces.

Writing Prompt: Answer the following questions on a lined piece of paper


1. True or False: A dark piece of paper will absorb more radiant energy from a light bulb than a white piece of paper.

2. In the video, why did the radiometer speed up when the light bulb was turned on?

3. What are two different examples of "radiant energy"?

4. Write a paragraph explaining the following concepts:

  • What is a radiometer?
  • How does a radiometer work?
  • A radiometer gets the energy it needs to do things from the light bulb. How is the relationship between the radiometer and light bulb similar to the relationship between the Earth and the Sun?