How Does a Crookes Radiometer Work?
By Science Decor | Published: 2026-10-23
Category: How-to Guides
Learn the science behind the Crookes radiometer (light mill). Discover how sunlight spins the vanes inside the glass bulb and what this tells us about solar energy.
Quick answer
A Crookes radiometer, also called a light mill, has a set of vanes inside a partial vacuum that spin when exposed to sunlight. The rotation is caused by a temperature difference between the black and silver sides of the vanes, not by light pressure.
Why the vanes spin: thermal transpiration, not light pressure
The Crookes radiometer is often mistaken for a device that runs on light pressure, but the real mechanism is thermal transpiration. The black side of each vane absorbs more sunlight and heats up, while the silver side reflects light and stays cooler. Air molecules inside the bulb bounce off the warmer black side with more energy, creating a tiny push that rotates the vane away from the hot side.
This effect only works because the bulb contains a partial vacuum—about 1/1000 of normal atmospheric pressure. At this pressure, molecules move freely without colliding too often, making the temperature difference matter. The rotation speed depends on the intensity of the light source, so a bright sunny window will produce faster spin than a cloudy day or an indoor lamp.
How to observe your Crookes radiometer working
- Place it in direct sunlight: Set the radiometer on a windowsill or table where it receives unobstructed sunlight. The stronger the light, the faster the vanes spin. Avoid placing it behind double-pane glass or tinted windows—these can reduce UV and infrared light and slow the rotation.
- Wait 30–60 seconds for the vanes to start: The vanes may not spin instantly. Give the black sides time to absorb heat and create the temperature difference needed for motion. If nothing happens after two minutes, check that the bulb is not in a shadow and that the glass is clean. Dust can block light.
- Observe the direction of rotation: The vanes spin with the black side trailing away from the light source. This confirms the thermal transpiration effect rather than light pressure, which would push the shiny side away. Try covering half the bulb with your hand. The vanes should slow or stop, then resume when uncovered—this shows the effect is light-dependent.
See the science in action
- Crookes Radiometer Blue: Our blue Crookes radiometer is not only an interesting decorative object, but it is also a useful tool for studying solar energy. Simply place the instrument in a sunny area and let it do its work. The blue-tinted glass adds a decorative element while still allowing you to observe the rotation clearly, making it a practical demonstration of solar energy conversion.
Crookes Radiometer Blue
