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DETECTION OF EXTRATERRESTRIAL LIFE
molecules, and that the molecules, their atoms and their electrons are always vibrating, then we can expect to find some of these vibrations to be in resonance with the vibrations of a portion of the electromagnetic spectrum.
If we want to study the absorption of ultraviolet light by substances, we need an ultraviolet spectrophotometer. This is an instrument that can separate ultraviolet light into bands of very narrow frequency ranges, and can measure the amount of ultraviolet passing through a substance. Thus, if we isolate a 
Figure 9.—Absorption spectra at pH 1; Alanylglycylglycine (tripeptide).
