Light engine


The light engine (radiometer) is composed of a glass bulb, filled with low pressure gas and containing a turning wheel with alternating black and white blades. If you light up the bulb, the wheel start to be in motion.


Switch on a spotlight and point it towards the bulb: the light is reflected by the white blades while adsorbed by the black ones; thus the temperature of the black blades gets higher than that of the white ones. From the kinetic theory of gases, we know that the mean kinetic energy of a particle increases with temperature: when a molecule of gas hits the dark face of a blade, it bounces back with higher speed than it would have when hitting a white blade.

Switch on the light!
In terms of forces, we might say that the particles hitting a white blade get a lesser impulse than the ones hitting a black blade. By the principle reaction, the impulse given by the gas particles to the white blades is smaller than that given to the black ones.
A curiosity: even an expert physicist may be induced into thinking that the turning of the wheel is due to light pressure (which is really present). In that case, from a more careful reasoning, the wheel should turn to the opposite direction than the one we observed.