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Crookes Radiometer - Astromedia

893.RMT

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The Crookes Radiometer, also known as a “light mill”, consists of a partly evacuated glass bulb, inside of which a set of vanes is suspended on a vertical axle. One side of the vanes is black, the other silver coloured. When exposed to bright light, the vanes start rotating about the axle. 

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Is this product normally on display at Grovers? Yes - But Check First If Making A Special Journey!

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Most people think that this is due to photon radiation pressure. Nevertheless, that can’t be the reason since the vanes turn so that the dark sides retreat from the source of light. If radiation pressure would be the reason for the rotation, they would rotate in the opposite direction since the radiation pressure on the silver sides is twice as large (photons being reflected instead of absorbed). The actual reason for the rotation is that the black sides get warmer than the silver ones. The gas molecules inside the glass bulb are heated by the black sides of the vanes, bounce off them and so, according to Newton’s third law, accelerate them in the opposite direction. The Radiometer is in fact a heat engine!

Our Crookes Radiometer is a beautiful glass instrument and never fails to fascinate!

(The radiometer is made from thin glass and children should only handle it under adult supervision!)

Height: 19cm, Diameter: 6cm

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