Hi There Am training to be science teacher at secondary school. A few issues have arisen for me lately and would like clarification: Issue 1: have been studying electromagnetic spectrum with my year 10 class. We have discussed Herschel's experiment in which he discovered IR radiation by passing sunlight through prism and placing thermometer at the point beyond red light. According to the class' textbook, the thermometer rose more in temperature when placed in the IR area than in any part of the visible spectrum. My issue with this: The sun is approximately a blackbody with T_eff roughly 6000K. This means that peak energy emission is in green light. Surely then the thermometer would have the highest increase in energy when placed in the green part of the spectrum? Is this a case of the school textbook oversimplifying or am I missing something? Issue 2: relates to UV light. I bought UV lights (LED I think) for my class. When we use them, there is a significant amount of blue light emitted. Why is there so much blue light? Is it because the glass covering over the LED is slightly fluorescent and so some UV gets degraded to blue light as it passes through glass? Or is the bandgap in the LED such that it covers a large enough range of wavelengths so that it goes from the bottom end of blue into ultraviolet... ? Note, the package says the wavelength range of the pen is something like 380-360nm... (These figures are from memory so might be a little off). Assuming figures are correct though, it is fair to assume that the blue light that can be seen is roughly 380nm (although the standard figure for vision cutoff is quoted as 400nm!). Also, if you look at UV lights used for disinfecting water and sewage, it can be seen that there is quite a bit of visible light coming off these as well! Again my question is: why isn't it possible to have lights that emit solely in UV - why is there this fluorescence even when the purpose is not for illumination? Is it simply a safety thing - a way to let people know when the lights are on or off? Or is it a technical difficulty with producing lights that emit solely in UV? Apologies if these points are overly verbose!