Measurement of the intensity of electromagnetic radiation in the visible spectrum transmitted through a solution or transparent solid. It is used to identify and determine the concentrations of substances that absorb light of a specific wavelength or colour according to Lambert's law, which relates the amount of light absorbed to the distance traveled through the absorbing medium, and Beer's law, relating it to the concentration of absorbing substance in the coloured solution. A photocell is often used to measure the amount of light transmitted through a glass tube containing the solution to be analyzed; the result is compared with results from a similar tube containing solvent alone. Most elements and many compounds, in appropriately treated samples, may be identified by colorimetry or spectrophotometry, a closely related technique.
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