I lit a candle so that we could see what we were doing in the Secret Cave in Langstrath. So why is a candle flame yellow? It produces a lot of small soot particles which are heated to a couple of hundred degrees. Then they act as tiny black-bodies. A black-body is a technical term in Physics for a perfect emitter or absorber of electromagnetic radiation. They all emit across a continuous spectrum but the colour that you see comes from the wavelength that produces the peak in the spectrum. Wien's Law says that wavelength x temperature (in Kelvins) = 0.0029. So if temperature increases, wavelength decreases. At hot temperatures, that wavelength is enough to be visible. The hotter it gets the more it moves from red through to blue. So a yellow flame is not actually that hot.
