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A Radiometer is a freely rotating 4-vaned propeller inserted in a vacuum in a light bulbshaped jar. Developed to measure the intensity of radiant energy or heat, the radiometer will help you understand the principle of energy conversion and show how heat and mechanical energy are products of energy conversion. The opposite side of each vane of the propeller is coated with white and black color. As the light hits the radiometer, the lightened side reflects the light while the darkened side absorbs it. As the darkened side absorbs the radiant energy, a difference in temperature develops between the vanes. Since the temperature of the gas molecules between the vanes are closer to the blackened part, the molecules start to move faster. These freely moving gas molecules bounce off the darkened side with a great deal of energy. So it starts to rotate.