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A silver-white metallic element, iridium has 77 electrons around its nucleus and, therefore, is number 77 in the Periodic Table of Elements. (We used to refer to iridium as having an atomic weight of 77, but that characterization is only relative and not absolute; the term, therefore, is obsolete.) The name is from the Greek iridis, translating as rainbow, due to the fact that when iridium is dissolved in hydrochloric acid, its salts are of various colors of the rainbow. Iridium is a component of extremely hard, corrosion-resistant alloys used in pen nibs, jewelry, surgical instruments, watch and compass pivot bearings, electrodes, and chemical crucibles. At one time, the official measurement of the meter was recorded as the distance between two fine lines engraved on a platinum-iridium bar kept at the International Bureau of Weights and Measures in Paris. See also Iridium.

InfraRed. Infrared light is in the frequency range of 300 GHz