In message , Bjoern Feuerbacher
writes
4B wrote:
What about those small propels that "mysteriously" spin when they are put
close to a source of light. Is not the momentum of the photons that make
them spin?

1) Don't top-post, please.
2) *Lots* of photons are acting there together.
3) You need an apparat with very few friction forces for this to work.
4) It is constructed especially to maximize the effect, by making one
side black and the opposite one light-reflecting.

And the cheap ones sold commercially spin the wrong way since the vacuum
in them is relatively poor. Warm gas molecules recoiling on the black
side of the vane dominate the forces acting.

When you pump them down to much lower pressures the recoil of photons
off the mirror side eventually dominates. It is quite a fun demo.

Regards,
--
Martin Brown