OUR LIGHT MILL IN ACTION

DESCRIPTION

“The Crookes radiometer, also known as the light mill, consists of an airtight glass bulb, containing a partial vacuum. Inside, a set of vanes is mounted on a spindle. The vanes rotate when exposed to light, with faster rotation for more intense light, providing a quantitative measurement of electromagnetic radiation intensity.”

http://en.wikipedia.org/wiki/Crookes_radiometer

EXPLANATION

The motion of the air molecules in the radiometer from the hotter (black) to the colder (silver) surfaces of the vanes contributes to the rotation of the vanes. Molecules hitting the black side get heated up slightly and bounce off with higher energy. Due to the momentum conservation, this causes the vanes to recoil in the opposite direction. 

 Certainly this is a simplistic way to explain how solar radiometer works. The full detailed explanation was given by Reynolds in 1879. He used the term “thermal transpiration”. Let's try to explain a little bit more.  When the solar radiometer is placed under the sunlight, heat is being introduced in the form of light. Each of the four vanes has a silver

reflective side and darker black side. That  darker black side tends to  absorb light and hence heat which means we  have one side of the vane  a bit hotter than  the other side. However,nature likes to even things out by sending colder air on the silvery side around to the darker side

cool it off and  when it does that the balance of the gases changes.Air pressure builds up on the darker side while decreasing on the silvery side . As the cool air moves air particles move around to the warmer side and sometimes they displace some the warmer molecules which go to the other site. Now by definition warmer air molecules have more energy and are more excited , so they strike the vane with more speed. They  actually strike the black side to the vane  while on the other side there is  very little resistance because of the lower air pressure there. So the vane spins  around with the dark side leading.

Note that the first explanation made by Crookes himself, was based on the air pressure fact.  However, while light does exert a pressure, it's too tiny to cause the effects seen in the radiometer. Moreover, if this was true, then the reflective side of each one vane

should have been pushed along and the vane would have spined

in the opposite than the observed direction.