Here's something interesting that one of my students (Julia RB) spotted today. We've been doing a design lab using "the worlds smallest solar powered car" and a lamp. It's all related to Earth day (which was yesterday). The idea was that all classes would do something with an environmental theme so I started with an intro to solar power. I found this interesting map on Wikipedia showing the amount of land that would have to be covered to collect enough solar energy to meet today's needs. Surprisingly little. The little cars actually don't use solar power directly they use light from a lamp that is produced from hydroelectricity, (in Norway) which comes from water that landed on top of mountains, due to the rain cycle which is driven by the sun. Anyway that's a bit off topic, the point that Julia was making was that after exposure to the lamp the photovoltaic cells change colour. This can be seen clearly in the photo, the one on the right has gone blue after exposure to light. Here's my theory: The photovoltaic cells are covered with a thin non-reflective layer. This is a thin film, light reflected off the top and bottom of the film interferes destructively cancelling out the reflection. This layer is very thin so the path difference is small however the waves are out of phase because the top wave undergoes a phase change of π on reflection. Now if the film gets hot, as it does under the lamp, it  expands and the path difference becomes bigger until it equals 1/2 λ for blue light. Blue light will now interfere constructively. Of course it could be due to something completely different. Talking of something completely different, in one of my other classes two students found something else interesting about the cars.