A white/black fan placed in a sealed flask spins when exposed to light!
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Teachable Topics:
- thermal engine
- radiation
Theory:
Most radiometers are sold with the explanation that they will spin when exposed to light due to the "photon pressure". But it becomes quickly clear that just photons alone are insufficient.
In this experiment, we expose the radiometer to various light sources, including LED, red laser, ultra-violet, infra-red, and incandescent bulb. We very quickly see that it is only sources that also have a significant thermal output (IR and incandescent bulb) that will cause the radiometer to spin.
This is due to the pressure caused by the black side of the vanes to heat up more efficiently than the white (or mirrored) side. It is important to note that if it was incident photons that caused spinning, the radiometer would spin in the other direction!
Apparatus:
- Various light sources
- Radiometer
Procedure:
- Place the Radiometer apparatus on a flat surface.
- Illuminate the radiometer with the various light sources and notice the effect.
Tips:
- This apparatus is very delicate, and should be handled with great care.
