Shining light on Beer’s law

Abstract

Beer’s law describes the diminution in intensity as light passes through an absorbing medium. Photometry, the primary application of the law in chemistry, is used to deduce the concentration of a light-absorbing component from the decrease in the intensity of monochromatic radiation during passage through a known length of the medium. This article describes the history and nomenclature of the law, as well as its mathematical basis. It goes on to show how the law becomes modified if the absorbing species not only absorbs the light but is also slowly destroyed by it. An intriguing feature is that the same function—the so-called “bleaching function”—describes not only the way in which the light intensity declines with distance, but also the dependence of concentration on time.