Crookes radiometer

The radiometer is made from a glass bulb from which much of the air has been removed to form a partial vacuum. Inside the bulb, on a low friction spindle, is a rotor with four vertical lightweight metal vanes spaced equally around the axis. The vanes are white on one side and black on the other and When exposed to sunlight or laser light the vanes turn with no apparent motive power. The dark sides absorbing the radiation and the white sides reflecting the radiation causing temperature difference between both sides of vanes. The higher temperature the more parcicles hiting the black side than the white side and the rotor strt turn. (Cooling the radiometer causes rotation in the opposite direction)

Originaly Crookes incorrectly suggested that the force was due to the pressure of light, but if this was true the the vanes should rotate the oposite direction (fotons would transfer more momentum to the white side than the black side).