Crooke's Radiometer

I remembered this experiment and it turned out we had it in the cupboard. It's like a 4-paneled weather vane in a partial vacuum. When you shine a light on it, the vane spins. I gave out what I understood to be the explanation, but I was wrong! I said that the momentum of the light photons, calculated by De Broglie's equation, changes on impact with the vanes, so you get an impulse.Thinking about it now, I can see a flaw in my own reasoning. The impulse force would need to create a torque but the light shines equally on both sides as far as I can see. The shiny side would reflect light so there would be a greater change in momentum  and thus a bigger impulse on the shiny side. This would mean it turning anti-clockwise. In fact, it turned clockwise. The real explanation is thermodynamic. I'm still digesting it, but it's here, with a film of it in action https://en.wikipedia.org/wiki/Crookes_radiometer