Radiometra

Radiometra, a sly goblin, holds a glowing glass bulb with four vanes inside, the light-mill that spins in sunshine, and that fools almost everyone about why.

The Physics

A Crookes radiometer has four vanes, each black on one side and silver on the other, inside a bulb pumped down to a partial vacuum. In light, the vanes spin with the dark sides retreating. If light pressure drove it, the reflective sides would be pushed harder (a reflected photon gives twice the push of an absorbed one) and it would spin the other way. Instead, the dark side gets warmer, and the thin gas does the work. James Clerk Maxwell (Phil. Trans. R. Soc. 170, 231, 1879) and Osborne Reynolds, in his study of thermal transpiration (Phil. Trans. R. Soc. 170, 727, 1879), explained it: gas creeps around each vane's edge from the cold side to the hot side, and the reaction pushes the vane towards its cooler, silver side.

dark side warmer ⇒ vane moves dark side trailing

• dark side: absorbs light and heats up
• silver side: reflects light and stays cooler
• gas: thin enough for molecules to travel far between collisions, but not a hard vacuum

It needs just the right pressure. In a hard vacuum, light pressure alone is far too weak to turn the vanes against friction, and it would turn them the other way.

Honest caveat: The full picture combines thermal transpiration and forces at the vane edges; the simple idea that hot gas molecules bounce harder off the dark face does not explain it on its own.

On the sticker

Radiometra cups a glowing bulb with four crossed vanes; which face is dark is not distinguishable in the art. Printed text: RADIOMETRA with the line NOT LIGHT PRESSURE. Pixel-art kiss-cut vinyl sticker, white, in 2″, 3″, 4″ and 6″ sizes.