I had PSSC physics in HS, and saw the PSSC films, one of which was exclusively devoted to the radiometer, and the hypothesis that it’s rotation was caused by “photons bouncing off the blades” like pellets.
An analysis was given indicating that the white, reflecting side should recieve more force ( because the pellets bounce off ) than the dark, absorbing side. I well remember Jerrold Zacharias pointing at the spinning vanes and saying, “... so it should spin this way” pointing in the opposite direction to the actual spin, then saying, “... clearly something is wrong.”
The “true” explanation is given that the light heats the dark sides, and the adsorbed gas molecules from the “evacuated” bulb are expelled with greater speed than from the cooler light sides.
The film continues with a demonstration of the very much smaller effect of momentum transfer by reflection of photons. A gold leaf is suspended in a much higher vacuum, and a very intense beam of light is focused on it, causing it to twist ever so slowly.
... some things, you just can’t forget!
Your comment is quite correct as photonic pressure does indeed provide a force applied to all objects in orbit exposed to direct sunlight. The result is the orbital path of such objects changes over time and requires course corrections by firing thrusters. When the thruster fuel (mass) is used up that vehicle will eventually cease to be operational.
The pressure of light photons is very slight and has been postulated as a propellant force for deep space exploration. Such craft would have to deploy light sails made of very thin metalized Mylar and rigged with Kevlar cords after launch.
The sails would probably be configured something like a parachute jib on a water going sail boat. Manipulating the Kevlar rigging would allow course corrections of the vehicle with no consumption of mass. The sails would have to be huge (acres?) to move a photon powered craft in any reasonable time frame. Tracking the craft from earth with a large laser beam would allow additional boost for a push during the initial phase of the journey.
However, the photon pressure is not causing rotation of the radiometer directly. If you look at the pictures you should notice that each vane is twisted slightly with respect to the center support. Also note that each vane is white on one side while the reverse side is black. Light shining on the radiometer heats the black side of the vanes but has a very slight effect on the white sides. The difference in heating creates a slight convection current of the gas contained in the globe. The motion of the gas is enough to spin the rotor, not the result of light directly.
Regards,
GtG