Phosphorescence

Phosphorescence is a type of photoluminescence related to fluorescence. When exposed to light (radiation) of a shorter wavelength, a phosphorescent substance will glow, absorbing the light and reemitting it at a longer wavelength. Unlike fluorescence, a phosphorescent material does not immediately reemit the radiation it absorbs. Instead, a phosphorescent material absorbs some of the radiation energy and reemits it for a much longer time after the radiation source is removed.

In a general sense, there is no distinct boundary between the emission times of fluorescence and phosphorescence (i.e.: if a substance glows under a black light it is generally considered fluorescent, and if it glows in the dark it is often simply called phosphorescent). In a modern, scientific sense, the phenomena can usually be classified by the three different mechanisms that produce the light and the typical timescales at which they emit light: fluorescence, triplet phosphorescence, and persistent phosphorescence. Whereas fluorescent materials stop emitting light within nanoseconds (billionths of a second) after the excitation radiation is removed, phosphorescent materials may continue to emit an afterglow ranging from a few microseconds to many hours after the excitation is removed.

There are two separate mechanisms that may produce phosphorescence, called triplet phosphorescence (or simply phosphorescence) and persistent phosphorescence (or persistent luminescence):

Everyday examples of phosphorescent materials are the glow-in-the-dark toys, stickers, paint, and clock dials that glow after being charged with a bright light such as in any normal reading or room light. Typically, the glow slowly fades out, sometimes within a few minutes or up to a few hours in a dark room.^{[further explanation needed]}

The study of phosphorescent materials led to the discovery of radioactive decay.

The term phosphorescence comes from the Ancient Greek word φῶς (phos), meaning "light", and the Greek suffix -φόρος (-phoros), meaning "to bear", combined with the Latin suffix -escentem, meaning "becoming of", "having a tendency towards", or "with the essence of". Thus, phosphorescence literally means "having a tendency to bear light". It was first recorded in 1766.

The term phosphor had been used since the Middle Ages to describe minerals that glowed in the dark. One of the most famous, but not the first, was Bolognian phosphor. Around 1604, Vincenzo Casciarolo discovered a "lapis solaris" near Bologna, Italy. Once heated in an oxygen-rich furnace, it thereafter absorbed sunlight and glowed in the dark. In 1677, Hennig Brand isolated a new element that glowed due to a chemiluminescent reaction when exposed to air, and named it "phosphorus".

In contrast, the term luminescence (from the Latin lumen for "light"), was coined by Eilhardt Wiedemann in 1888 as a term to refer to "light without heat", while "fluorescence" by Sir George Stokes in 1852, when he noticed that, when exposing a solution of quinine sulfate to light refracted through a prism, the solution glowed when exposed to the mysterious invisible-light (now known to be UV light) beyond the violet end of the spectrum. Stokes formed the term from a combination of fluorspar and opalescence (preferring to use a mineral instead of a solution), albeit it was later discovered that fluorspar glows due to phosphorescence.