Crooke's radiometer with flashlight, arc lamp or IR source.

Crooke's Radiometer with white light source or IR source. Light from a flashlight,arc lamp, slide-projector, laser pointer or infrared source illuminates the Radiometer. The vanes of the Radiometer turn. The device has a slight amount of gas inside. Each vane is black on one side and silvered on the other.The black side absorbs IR and heats up, heating the adjacent gas. The temperature difference between the two vanes creates a convection from the light side to the dark side, and by Newton_s 3rd Law, the vanes move in the opposite direction. CONVECTION.
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This is not a demo that participants are allowed to pick up and interact with. The main purpose of having it is to promote a discussion about radiation pressure vs thermal expansion of gasses. Most people have an innate conception that the Crooke's radiometer works because light is pushing on it, not because of thermal differences between the sides of the veins. Some leading questions to help people understand could include "which would you rather have in hot weather, a white car or a black car?" As always when you present a demo make sure you ask the participants questions before you demonstrate and after. Think about the concepts involved beforehand; having a friend ask you questions about it can be very revealing as to if you understand it well or not. Whenever you ask a question count to five (or at least three) in your head waiting for an answer before you stay anything else.