No pot of gold
Today my SL class were doing a variety of practicals on diffraction. Most did single slit diffraction but a couple were doing diffraction from a CD. After some time trying the CD group asked for help because they couldn't see the rings that I promised on the worksheet and nor could I. I was sure that it had worked before so I returned to the lab after class to try again. This time it worked perfectly and I even took the magnificent photo you can see here. The problem earlier was we were holding the CD too far away (if I'd read my own worksheet I would have realised). You tend to hold the CD where you can see it in focus but actually it should be much closer. You're not actually looking at the CD at all but the reflected light.
On getting home I decided to try to analyse the photo. The first thing I noticed was that the colours don't seem to be in the right order (remember I'm colour blind). Why is blue diffracted more than red? Well this is because the outer blue line is the first part of the second order spectrum. The first order blue ring is the inner one. I analysed the radius of the rings in loggerPro and found that the radius of the outer blue ring was pretty close to 2 x the inner one, which is what it should be if the angles are small.