1852

Fluorescence

George Gabriel Stokes (1819–1903)

The property of fluorescence was observed as early as the sixteenth century: some water extracts from exotic woods glowed blue around the edges of their containers when exposed to light and viewed from the right angle. Over the next few hundred years, many other substances were found with similar curious properties, including the mineral fluorspar and glass colored with uranium salts. With the discovery of ultraviolet light at the beginning of the 1800s, it became clear that many of these materials were somehow absorbing ultraviolet light’s invisible rays and reemitting visible light instead.

Irish physicist Sir George Gabriel Stokes was the first to work this out (partly by using solutions of quinine, which has a fairly intense blue fluorescence), and he named the property after fluorspar. The mechanism for this glow was not immediately understood, though, because a full treatment needed an understanding of quantum mechanics. We now know that some materials can have some of their electrons excited by absorbing higher-energy radiation, and the excited electrons then fall back to their original ground state by emitting energy in the form of light (almost always light of a longer wavelength, and therefore more visible to us, than what was absorbed).

Fluorescent compounds abound, and many of them have been made deliberately. Fluorescent dyes are well known for their strangely glowing colors, a result of their producing more visible light than our eyes expect them to be able to. Although it’s probably most common to think of fluorescence in relation to highlighters or orange safety vests, fluorescence is used in biomedical research as well: specific wavelengths are used for excitation and other specific ones are emitted, allowing substances (even inside living cells) to light up on command, completely distinct from their background. Fluorescent labels are even used to mark cancerous tissue in human patients for surgery. Spanning the visible spectrum, these fluorescent molecules have provided huge amounts of information that otherwise would have been hidden.

A collection of fluorescent minerals under ultraviolet light. Some of these are also colorful under ordinary light, but not to this extent!