Crooke's Radiometer
Crooke's Radiometer
Investigating the energy and impulse of an electromagnetic wave Invented by Sir William Crooke, the vanes in the highly effective radiometer rotate when exposed to solar radiation.
The reason for this rotation can open up much debate and students should be encouraged to develop their own explanations. Some will state that the device relies upon the difference in absorption of impulse between the black and metal vanes. Others may think there is a difference in temperature of the vanes. This device has been designed with black vanes that absorb electromagnetic radiation and metal vanes that reflect. Students need also to remember that black sides get warmer than metallic ones.
experiment: Black body - Heating up the vanes
The right solution of the puzzle concerns heat considerations. We know that the black surface is warmer than the shiny one and that gas molecules will recoil faster from a hot surface.
The slight difference in molecule recoil is what causes the device to spin. The other proposal involving photon absorption by the black vane and reflection by the metal side is physically correct, but negligible with respect to the main effect described above. With a light sensor is also possible to measure the frequency of rotation and calculate the angular speed of the radiometer.
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