Rave glasses
I distinctly remember saying to the class, "look at the light not the camera". They ignored me of course and although I got a nice photo of my class it didn't really illustrate the point I was trying to make. Next time I'll wear my glasses, reading glasses that is not spectrum glasses. Actually now I look at the photo I can see why they weren't looking at the light source. It wasn't switched on and wasn't even pointing towards them. The point was that these spectral glasses are great when introducing line spectra. I used to pass round the 4 proper diffraction gratings but these are cheaper and work just as well. You can get them from eBay for under a pound.