Converting light into heat/movement, reflection vs absorption

Question

at school we had a small device in a glass dome it had 4 plates every plate was blackened on one side and silver colored on the other side. when the sun did shine on one side of it, it would start to spin because the air at the black side would heat up and expand more than the air at the normal metal color side. however this device used sun power and is rather simple and straight foreward in function. however since somehow many people use reflective surfaces when trying to convert laser light into movement,I wondered what kind of effect and difference you have between light reflection and light absorption in relation to turning light into movement.

is that reflection stuff just to make it work in space, or is there a logical reason to on earth in the earth atmosphere use reflection instead of absorption (to generate heat and make the air expand) to turn light into movement?

since most likely things like thermal conductivity and wavelength also play a role in this for this fictive scenario there is a IR led as light source. and the materials to add force to are aluminum, aluminum coated black in wood fire or with carbon powder, black paper and optionally some other materials which might have more interesting physics effects behind them.

Answer 1

This is a fun question, because the behavior of your referenced toy depends critically on the existence of air inside the bulb. If the bulb were evacuated, then momentum conservation laws will show that the plates will spin in the opposite direction. Photons hitting the white plates are reflected and thus impart 2X momentum compared with photons hitting the black plates and being absorbed.

The design of "solar sails," then, selects materials which are highly reflective so as to gain as much momentum as possible from the solar irradiance.

And of course, you could use an absorber to heat a propulsion mass (maybe make water boil & use the pressure generated as a steam enging) but then you have to carry that mass with you in the first place.