Buzz
Inside a Crookes radiometer, you'll find a glass bulb with a partial vacuum, housing a spindle with mounted vanes. Richard Newstead / Getty ImagesMain Points to Remember
- The Crookes' radiometer features a sealed glass bulb with a partial vacuum and vanes that rotate upon exposure to light.
- This rotation is driven by thermal transpiration, where light heats the blackened side of the vanes, expelling air more rapidly and pushing the vane away from the light.
- The radiometer's functionality relies on a partial vacuum; in a complete vacuum or without one, the vanes remain stationary due to the absence of air drag or the impossibility of thermal transpiration.
Inside a Crookes' radiometer, four vanes are suspended within a glass bulb containing a partial vacuum. When light is directed onto the vanes, they begin to rotate, reaching speeds of several thousand rotations per minute under intense sunlight.
The presence of a vacuum is crucial for the radiometer's operation. Without a vacuum (i.e., if the bulb is filled with air), the vanes won't spin due to excessive drag. In a near-perfect vacuum, the vanes only rotate if supported frictionlessly. In such a scenario, photons striking the silver side of the vanes exert a minimal force, causing rotation.
In a partial vacuum, a phenomenon known as thermal transpiration occurs along the vane edges, as explained on this page. This effect creates the illusion that light is pushing against the black surfaces, causing the black side of the vane to move away from the light source.
Explore these resources for further information:
- How does a light-mill work?
- Science Museum: Crookes' Radiometer
- The Radiometer
- Vacuum basics
