What if light could push physical objects? Sounds impossible right? Well, a German toy has already done it. A fan-like structure inside a light bulb mounted on a stand. Which has quite a strong vacuum inside but not that strong which can create inessential drag for the fans. Which is why air cannot blow the fans at all, rather like a “light mill”, it’s only moved by lights.
But how light moves it? It is said that just like wind pushes a windmill, the radiometer is pushed by the photons of light, when the photons of light hit the fans, it starts to move. Also, there are many theories about how this crooks’ radiometer works.
First theory says that when you turn on the light, the fan moves in such a way that it looks like the light is “pushing” the black side of the fan. Which later was abandoned because if the photons are absorbed from the black side and are reflected at the silver side then won’t the fan move in the opposite direction?
Then, another theory came, stating that the absorbed radiation heats the gas inside the radiometer and the heated gas creates a push on that side. The greater the heat, the more this backside would get pushed and it would spin faster. But then, later, this theory was proved to be wrong as well. But what really happens?
The reason is thermal transpiration. The edges of the fans are plays the major role here and not the faces. When the air is warmer, the fans move along the surface from colder to the warmer side through the edges because of the temperature gradient. Just pressure difference does the whole thing; if the negative temperature gradient then it would move in the opposite direction.