Incandescence is a phenomenon that occurs when a substance emits light when heated.
When the atoms that make up a substance are heated, they absorb a significant amount of energy. The electrons, which are located on electronic layers (as established by the physicist Niels Bohr), move away from the nucleus. When the electrons lose this energy, they return to their original electron layer, emitting the excess energy in the form of visible radiation (producing light) or infrared radiation (emitting heat).
Among the common objects or phenomena that produce light by incandescence, here are a few examples:
- incandescent light bulbs
- certain heating elements such as in a toaster or on a cooker;
- molten lava
- fire;
- the Sun.
Luminescence is the production of light by materials at low temperature.
Compared with incandescence, which produces light through the heat released, luminescence is said to produce cold light, because the light is produced without intense heating of the material.
But whatever the method of production, the phenomenon can always be explained in the same way: firstly, the atoms of the luminous substance receive energy in the form of chemical, luminous, electrical or mechanical energy. Then, the electrons in these atoms, which have an additional amount of energy, convert it into light energy.
There are several ways of producing light by luminescence.
Fluorescence is the immediate re-emission in luminous form of energy absorbed by a substance in a specific colour.
A fluorescent substance will only produce visible light when it is stimulated. In total darkness, fluorescence will not be observed over a long period, unlike phosphorescence.
Fluorescent orange waistcoats on construction sites have the property of absorbing daylight, car headlights or any other light and instantly transforming it into orange light. As a fluorescent colour transforms almost all the energy received into light of its own colour, it appears brighter than other colours, which only reflect part of the energy received.
Fluorescent tubes are another example of fluorescence. These tubes contain a mixture of argon and mercury. When an electric current is passed through this mixture, it produces ultraviolet radiation that strikes the walls of the glass tube. These walls are coated with a white powder, which is used to transform the radiation inside the tube into fluorescent white light, which is however different from non-fluorescent white light.
Road signs also emit light through fluorescence.
A phosphorescent substance absorbs the surrounding light and re-emits it over a long period.
Unlike fluorescent substances, which immediately emit the light received, phosphorescent substances can conserve this energy and produce light over a very long period. It is therefore possible for a phosphorescent substance to emit light in the dark, whereas this is not the case for a fluorescent substance, which must be illuminated in order to re-emit light.
Some watch hands absorb the light they receive as long as the environment is lit. When night falls (or the lights are switched off), the hands slowly emit the energy received in the form of light, allowing them to be seen for as long as the stored energy is not exhausted. This type of watch could, for example, enable a diver to see the time in the depths of the water, where light is scarce.
Some rocks have phosphorescent properties.
Phosphorescent substances can be used to create works of art.
The northern lights are also a phenomenon of phosphorescence.
Chemiluminescence is the production of light as a result of chemical reactions that produce energy in the form of light.
The reactants in this type of reaction exchange electrons with each other: it is during this exchange that the electrons, once excited, will return to their normal state, producing light.
Glow sticks contain cyalume which, when in contact with the hydrogen peroxide in the central tube, produces light.
Luminol used by the investigators can detect the presence of blood that would be invisible to the naked eye.
Bioluminescence is the production and emission of light by living organisms, which convert chemical energy into light energy.
Living organisms use bioluminescence to attract other living organisms or to communicate with each other. In some cases, bioluminescence can be used as a means of protection, since it can repel predators.
Fireflies produce yellow or green light thanks to an enzyme called luciferase.
Dinophytes are protists (single-celled organisms) that use their light to attract prey.
Electroluminescence is the production of light by the passage of an electric current through a material.
Electroluminescence uses electrons to excite certain particles, which then produce light.
Backlighting is used to improve the display on liquid crystal screens.
Light-emitting diodes are increasingly used in everyday life. Light bulbs and different types of headlights use this type of technology to produce light.
Triboluminescence is the production of light from mechanical energy.
Triboluminescence occurs when the bonds in crystalline structures, such as those found in diamonds or sugar cubes, are broken.
When both sides of an envelope are peeled back in the dark, it is sometimes possible to see light.
