Where's Richard
The other day I was playing with some old CDs. Sticking them to a spring so the end could be sensed by a motion sensor. While doing this I noticed that there were some interesting fringe patterns where the silver foil was peeling off the plastic, so I took a photo. When I looked more closely at the photo I saw something else interesting. The fluorescent light reflecting off the CD had formed a spectrum Richard of York Gave Battle in Vain, right? Red orange yellow indigo violet. But wait, where's Richard? I could be entering dangerous territory here since I'm colour blind and there could well be red in that photo but assuming there isn't here is the explanation: In a fluorescent tube excited gas molecules emit UV light that is absorbed by a coating on the inside of the glass tube. This coating (phosphor) absorbs the UV and emits visible light. But the phosphor only emits a narrow range of wavelengths so to get white light you need different chemicals in the coating. So the fluorescent tube in question did not have a phosphor emitting red light. Either that or because I'm colour blind I can't see it. I'll ask my wife what she thinks.