General principles of spectroscopy and spectroscopic analysis

where Z is the refractive index defined as the ratio of the speed of light in a vacuum to the speed of light in the given medium, date from this period. In more modern times, infrared, ‘‘beyond red,’’ radiation was discovered by William Herschel in 1800. He found that the temperature recorded by a thermometer increased from the violet to the red in the visible region. Going beyond the red he found the temperature continued to increase instead of decreasing if the light ended at the end of the visible spectrum. Recognition of the utility of the infrared spectral region for chemical analysis is credited to W.W. Coblentz and it was not until the mid-1900s that infrared spectroscopy became an established technique. Apparently, noting Herschel’s discovery of infrared radiation, Johann Wilhelm Ritter discovered ultraviolet radiation in 1801 by noting that silver chloride was reduced to silver metal when exposed to violet visible light and was even more efficiently reduced by radiation beyond the violet end of the visible spectrum.