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Andrew Matheson
Honourable Mention
2006 OAPT 4C Photo Contest
Light Waves and the Radiometer
There are two theories to how Crookes' Radiometer works, both spin in different directions. On way occurs only when there is a perfect vacuum, that is when the silver side moves away from the light. What happens is the black side absorbs the light waves, while the silver side gains twice the momentum of the black side because of the waves bouncing off it, as a result of Newton's third law. Therefore it rotates in the direction of the silver side.
The alternative explanation is when there is a partial vacuum, leaving some air molecules. Because black absorbs light waves it heats up along with the air molecules around it, causing them to move faster. The particles bounce off the black side with more momentum, so it rotates in the direction of the black side.