Energizing a Radiometer with an LED Flashlight

What light sources can you use to energize (and spin) a radiometer?

Time: 3 minutes


  • Radiometer
  • LED Flashlight (or other)


  • Find a spot away from a strong light source to place your radiometer.
  • Next, turn on your flashlight and bring it close to the radiometer.
  • TIP: You may need to jiggle the radiometer depending on the power of your light source. The jiggle with actually help overcome the inertia of the paddles within the radiometer.
  • Experiment with different light sources and different distances.


  • What did you see? Did the radiometer spin?
  • Which direction did it spin? Which side of each paddle (white or black) spins away from (or towards) the light source?


When I first saw a radiometer, I thought the light from the sun was ‘bouncing’ off the white surface and pushing the white side away from the light. Then, upon further inspection, it became clear that the black side was moving away from the light. It is very interesting physics. The radiometer absorbs electromagnetic energy (light energy) and because the black absorbs more energy than the white side, the particles on the black side get excited and become warmer. Now, we have a warm side (the black side) and a cool side (the white side). The warmer side has a slightly higher air pressure than the cooler side (think of gas expanding when it is warmed up because the molecules are jiggling more and hence need more elbow room). This differential between high pressure on the black side and low pressure on the white side causes the molecules within the radiometer to flow from high to low pressure and as they do they move away from the black side which has the effect of pushing the black side away.

By the way, this type of radiometer is a Crookes Radiometer which was a very popular science toy to own in the 1870s! It is vacuum sealed so that there is very little resistance (wind resistance) inside of the glass bulb. Without this vacuum, the friction would be very hard to overcome.


Radiometer Video:


  • What happens with a stronger light source?
  • What happens when you move the light source closer or farther away?
  • Does the color of the light source matter? For example, does an LED flashlight act differently than an incandescent bulb flashlight?
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Coco Science

Coco is interested in STEAM with a particular interest in the A. She has a hamster named Marshmallow and likes to read, craft, cook and dance.