Fluorescence and phosphorescence are two mechanisms that emit light or examples of photoluminescence. However, the two terms don't mean the same thing and don't occur the same way. In both fluorescence and phosphorescence, molecules absorb light and emit photons with less energy (longer wavelength), but fluorescence occurs much more quickly than phosphorescence and does not change the spin direction of the electrons. Here's how photoluminescence works and a look at the processes of fluorescence and phosphorescence, with familiar examples of each type of light emission. Key Takeaways: Fluorescence Versus Phosphorescence Both fluorescence and phosphorescence are forms of photoluminescence. In a sense, both phenomena cause things to glow in the dark. In both cases, electrons absorb energy and release light when they return to a more stable state.

Fluorescence occurs much more quickly than phosphorescence. When the source of excitation is removed, the glow almost immediately ceases (fraction of a second). The direction of the electron spin does not change.

Phosphorescence lasts much longer than fluorescence (minutes to several hours). The direction of the electron spin may change when the electron moves to a lower energy state.

Photoluminescence Basics Fluorescence is a fast photoluminescence process, so you only see the glow when black light is shining on the object. Don Farrall / Getty Images Photoluminescence occurs when molecules absorb energy. If the light causes electronic excitation, the molecules are called excited. If light causes vibrational excitation, the molecules are called hot. Molecules may become excited by absorbing different types of energy, such as physical energy (light), chemical energy, or mechanical energy (e.g., friction or pressure). Absorbing light or photons may cause molecules to become both hot and excited. When excited, the electrons are raised to a higher energy level. As they return to a lower and more stable energy level, photons are released. The photons are perceived as photoluminescence. The two types of photoluminescence ad fluorescence and phosphorescence.

How Fluorescence Works A fluorescent light bulb is a good example of fluorescence. Bruno Ehrs / Getty Images In fluorescence, high energy (short wavelength, high frequency) light is absorbed, kicking an electron into an excited energy state. Usually, the absorbed light is in the ultraviolet range, The absorption process occurs quickly (over an interval of 10-15 seconds) and does not change the direction of the electron spin. Fluorescence occurs so quickly that if you turn out the light, the material stops glowing. The color (wavelength) of light emitted by fluorescence is nearly independent of the wavelength of incident light. In addition to visible light, infrared or IR light is also released. Vibrational relaxation releases IR light about 10-12 seconds after the incident radiation is absorbed. De-excitation to the electron ground state emits visible and IR light and occurs about 10-9 seconds after energy is absorbed. The difference in wavelength between the absorption and emission spectra of a fluorescent material is called its Stokes shift. Examples of Fluorescence Fluorescent lights and neon signs are examples of fluorescence, as are materials that glow under a black light, but stop glowing once the ultraviolet light is turned off. Some scorpions will fluoresce. They glow as long as an ultraviolet light provides energy, however, the exoskeleton of the animal does not protect it very well from the radiation, so you shouldn't keep a black light on for very long to see a scorpion glow. Some corals and fungi are fluorescent. Many highlighter pens are also fluorescent.