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In this activity, students observe solar energy being transferred into thermal and mechanical energy using a radiometer. 

When light rays hit the vanes of a radiometer, the black sides of the vanes absorb the rays better than the white sides. This causes the black side to become hotter than the white side (thermal energy). When molecules in the air hit the vanes heat energy is transferred to them. The molecules that hit the black side gain more energy and hence recoil with greater force than those that hit the white side, causing the vanes to spin (kinetic energy). 

The solar energy is changed first to thermal energy, and then to kinetic energy.

A partial vacuum inside the radiometer glass allows the vanes to spin easily. The brighter the light, the more heat energy is transferred, and faster the vanes will rotate. 


  • Explain how solar energy can be transferred to thermal or mechanical energy.


  • Per Demo:

    Tip: Radiometers may be purchased from science supply stores or online.

Key Questions

  • What happens when you shine less light on the vanes? More light?
  • Can you aim the lamp so that the light only hits the white sides of the vane? The black side? Is there a difference in how fast the radiometer spins?
  • What is causing the vanes to spin?
  • Is the colour important?

What To Do

  1. Shine light on the radiometer and watch it move.
  2. Hold the light at different distances, and partially block the light.
  3. Have the students observe and record if, and how, this changes the result.


  • On a sunny day, experiment with wearing a black shirt and then a white shirt. Do you notice a difference in temperature?
  • Leave one thermometer on a black car and one on a white car. Is one car hotter than the other?

Other Resources

British Columbia Sustainable Energy | Solar Photovoltaic