Solar Mill
 

During a recent visit to the Science Center, I bought one of those little solar mills - like a light bulb, with vanes that spin when in sunlight. They're fascinating little devices, and I've been reading a little bit about how they work.

One of the wrong theories that abounded for a while was that it's the actual light particles that make the vanes spin. This was discounted because, amongst other reasons, it spins the wrong way for that to be the reason. One side of the vanes is black and the other side reflective.

The reasoning I read was that if it were light particles making it spin, it would spin by pushing the reflective side away, as opposed to the dark side. The light particles, being absorbed by the dark side and reflected by the reflective side would - and here's where I have problems understanding - would impart more of their energy when they were reflected, rather than absorbed.

This puzzles me greatly. It's like saying a bullet, when shot against a suit of armour, would strike it with more of a force when it reflected (bounced) off the armour, rather than being absorbed.

In the case it is absorbed, all of the bullets energy is transfered to the armour, and the bullet comes to rest. However, when it bounces off, the bullet is still moving, albeit in the opposite direction, and therefore still has energy which it didn't impart to the armour. How can the reflected bullet strike the armour with a greater force then?

Ringo

Re: Solar Mill
 

Here is the Explanation

-Gryph

Re: Solar Mill
 

aah, thats actually one of the explanations that I read. It's this line thats puzzling me, though..

"Light falling on the black side should be absorbed, while light falling on the silver side of the vanes should be reflected. The net result is that there is twice as much radiation pressure on the metal side as on the black. In that case the mill is turning the wrong way."

I know thats one of the older, wrong theories, but the idea of twice as much radiation pressure on the metal (reflected) side, as opposed to the black (where it is absorbed) puzzles me.

Re: Solar Mill
 

You got to think of it in terms of vectors of forces and preservation of energy. In order to have to a photon "bounce" off the silver side, you need to have a force greater than the force of the photon striking the silver plate for it to change direction because of preservation of energy. So the silver side is exerting the kinetic energy to make the photon bounce off where the black side exerts the same energy on the photon and it gets transferred to the black plate through absorption. This absorption is translated to thermal energy which causes the thermal disparity on the edges.

I hope that that is clear.

-Gryph

Re: Solar Mill
 

Here is a basic explanation of Radiation Pressure

-Gryph