Millie micro nano pico Book 10 in which Millie fights for the liberation of infrared photons. At the beginning of this adventure, Millie asks the electrons to explain the nature of a rainbow. This allows her to learn that photons have a double nature: they are both particles and waves, just like electrons. Their waves have different wavelengths, which determine their colour. Millie also discovers that there are invisible photons with wavelengths longer than coloured photons. She hears about the ones that carry heat—called infrared photons. This adventure ends with Millie worrying about global warming and the greenhouse effect, in which infrared photons are directly involved.