Caption: "The Crookes radiometer, also known as the light mill or solar engine, consists of an airtight glass bulb, containing a partial vacuum. Inside are a set of vanes which are mounted on a spindle. The vanes rotate when exposed to light. The reason for the rotation has been the cause of much scientific debate."

The Crookes radiometer is actually a form of heat engine. But how the heat energy gets turned into mechanical energy is hard to understand.

The Crookes radiometer can be used to measure electromagnetic radiation, but nowadays it is mostly a novelty item. There continues to be scienfitic interest in how it and variations on it work.

Credit/Permission: © Nevit Dilmen (AKA User:Nevit), 2010 / Creative Commons CC BY-SA 3.0.
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