This is a Crookes Radiometer. An old physics department staple. What does it do? On exposure to light or heat, the vanes turn on a low friction mount. Here a green laser (14 mW green and 9 mW infrared) starts it turning.
“Continue reading” for more on this interesting science toy.
Why does it work? It’s complicated. It is not light pressure as often touted. Even Einstein had something to say about it. The current accepted explanation from Reynolds is due to thermal heating and edge effects.
It was hard to stop the rotation inside the black tube due to the infrared radiation from the warmth of the tube.
You can even start one of these in darkness by cupping your hands around it. Apparently the hand warms the glass which which radiates in the long infrared.
Above is a close up of the central pivot point seen in sunlight.
A Blu-ray violet laser above at about 70 mW makes it turn well, as does an older smaller Blu-ray laser of perhaps 20 mW.
The green laser that works well in the top photo is a nominal 5 mW. Measured output is 25 mW, 14 mW green and 9 mW infrared.
Typical red laser pointers of 1 mW – 5 mW won’t work, nor will a 10 mW HeNe laser (Melles Griot).
Hence it takes about 20 mW to run this.
Moonlight, above, was magnified with a Fresnel lens to see if it would turn the radiometer but it didn’t. In sunlight (100,000 lux = around 1 kW/m2) this set up would put 300 W in to the Radiometer and melt it within moments. Moonlight at 0.25 lux will however result in only 0.75 mW. As the laser experiments above show, it takes about 20 mW to make it run. Hence I would need 30 times more power to make it work.
Related pages
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External links
Initial demonstration
Green laser and other experiments
Photo Date: Dec 11, 2011