PHYSICS DEPT., IMPERIAL COLLEGE / SCIENCE PHOTO LIBRARY PHYSICS DEPT., IMPERIAL COLLEGE / SCIENCE PHOTO LIBRARY
The photograph of the emission spectrum of sodium shows a single bright yellow line. In fact there are two emission lines, but they are so close together that the picture would have to be enlarged to a length of at least 2 metres before they became distinguishable. The yellow glow of sodium street lights is due to excited sodium atoms emitting light at this wavelength.
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