Light is something like raindrops. Each little lump of light is called a photon and if the light is all one color, all the “raindrops” are the same.
The photon gas, a box filled with little light lumps, plays a very important role in physics. As a source of black body radiation — collect the photons coming from a small hole in the box and sort them by energy — it put Planck on the road to quantum mechanics, and although his hope that electromagnetic radiation was the origin of irreversibility did not work out as he intended, it is well worth studying this idea. Here, however, I investigate the photon gas as a medium for a Carnot engine.