A sample of quicklime (calcium oxide), photographed through a diffraction grating to reveal its spectra (the frequencies of electromagnetic radiation emitted by the compound's molecules when they are returned to a lower energy state). Each colour band corresponds to a specific energy of photon released by electrons jumping to different energy levels. In the days before electric lighting, theatres used quicklime to make 'limelight' for the stage. This was done by heating a cylinder of quicklime with an oxyhydrogen flame; the resulting bright light is produced by a combination of incandescence and candoluminescence. This historic usage is still reflected in the expression 'in the limelight,' referring to someone who is in the public eye.
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