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Light - Fluorescence & Phosphorescence     

In order to understand the phenomena of fluorescence and phosphorescence, it is necessary to consider the structure of the atom. In simplistic terms, an atom consists of a nucleus surrounded by a cloud of electrons. These electrons occupy a series of orbits or shells of several diameters. Each shell represents one energy level, and the larger the diameter of the shell, the higher the energy level.

It has been found that an electron travelling in its orbit must have a precise amount of energy in order to remain in that orbit or shell. If for some reason, the electron loses some energy it will move to an orbit or shell closer to the nucleus. On the other hand, if the electron acquires extra energy, it will move to an orbit or shell farther from the nucleus.

All kinds of radiation, including ultraviolet, are forms of energy. In most cases, when ultraviolet light is directed at an object, the energy of the light is absorbed and turns to heat. Consider the effect on your skin after lying in the sun too long!

However, in some cases, when substances are exposed to ultraviolet light, extra energy is given to electrons, so that they jump to higher energy levels. This is illustrated in the diagram below:

The jump by an electron to a higher energy level leaves a gap in the electron shell just vacated. As a result, to maintain the electrical balance, an electron from a higher shell is pulled back to fill the gap. This is called spontaneous emission, since the electron moving to an inner shell releases a quanta of energy known as a photon. It is this energy we see as visible light or fluorescence. In actual fact, this process of energy exchange takes place rapidly, with many electrons jumping to higher or lower energy levels … so the visible light we see, is for all practical purposes, continuous. The visible light given off can be of almost any colour, depending upon the substance fluorescing, the wavelength of the ultraviolet causing the fluorescence, and the type/amount of activator (see below). The ordinary fluorescent light bulb is a widespread application of this phenomenon. The tube, itself, is basically a generator of ultraviolet energy, and the inside surface of the glass is coated with a fluorescent powder or phosphor, which fluoresces brilliantly, yielding visible light.

When the energy source, such as an ultraviolet lamp, which causes electrons to jump to higher energy levels in the first place, is removed, the electrons in most fluorescent substances settle back quickly into their balanced orbits. As a result there is no further radiation of visible light. However, in some materials, this return to normal orbits is a slow process, so the atoms continue to give off light while the electrons are returning to their normal state. This continued emission of light after the ultraviolet light has been removed is known as phosphorescence. Some materials will phosphoresce for only a few seconds, while others will do so for long periods (weak phosphorescence having been detected in some cases even up to several years).

Most materials are not fluorescent. Few chemically pure materials will fluoresce at all. However, in substances containing a small amount of chemical impurity, fluorescence may be activated. The amount and type of impurity present will determine the colour and intensity of the fluorescence. For example, the red fluorescent calcite from north-western New Jersey is activated by manganese. In this case, manganese should be present in a quantity of about 3% (definitely less than 5%, and more than 1%).