Solar Mill

[Ringo]

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