Nature of Light
Light Emission | Electromagnetic Waves | Photons | Light Sources | Light Detection

Sources of light differ in how they provide energy to the charged particles, such as electrons, whose motion creates the light. If the energy comes from heat, then the source is called incandescent. If the energy comes from another source, such as chemical or electrical energy, the source is called luminescent (see Luminescence).

Incandescence: In an incandescent light source, hot atoms collide with each other. These collisions transfer energy to some electrons, boosting them into higher energy levels. As the electrons release this energy, they emit photons. Some collisions are weak and some are strong, so the electrons are excited to different energy levels and photons of different energies are emitted. Candle light is incandescent and results from the excited atoms of soot in the hot flame. Light from an incandescent light bulb comes from excited atoms in a thin wire called a filament that is heated by passing an electric current through it.

The sun is an incandescent light source, and its heat comes from nuclear reactions deep below its surface. As the nuclei of atoms interact and combine in a process called nuclear fusion, they release huge amounts of energy. This energy passes from atom to atom until it reaches the surface of the sun, where the temperature is about 6000° C (11,000° F). Different stars emit incandescent light of different frequencies—and therefore color—depending on their mass and their age.

All thermal, or heat, sources have a broad spectrum, which means they emit photons with a wide range of energies. The color of incandescent sources is related to their temperature, with hotter sources having more blue in their spectra, or ranges of photon energies, and cooler sources more red. About 75 percent of the radiation from an incandescent light bulb is infrared. Scientists learn about the properties of real incandescent light sources by comparing them to a theoretical incandescent light source called a black body. A black body is an ideal incandescent light source, with an emission spectrum that does not depend on what material the light comes from, but only its temperature.

Luminescence: A luminescent light source absorbs energy in some form other than heat, and is therefore usually cooler than an incandescent source. The color of a luminescent source is not related to its temperature. A fluorescent light is a type of luminescent source that makes use of chemical compounds called phosphors. Fluorescent light tubes are filled with mercury vapor and coated on the inside with phosphors. As electricity passes through the tube, it excites the mercury atoms and makes them emit blue, green, violet, and ultraviolet light. The electrons in phosphor atoms absorb the ultraviolet radiation, then release some energy to heat before emitting visible light with a lower frequency.

Phosphor compounds are also used to convert electron energy to light in a television picture tube. Beams of electrons in the tube collide with phosphor atoms in small dots on the screen, exciting the phosphor electrons to higher energy levels. As the electrons drop back to their original energy level, they emit some heat and visible light. The light from all the phosphor dots combines to form the picture.

In certain phosphor compounds, atoms remain excited for a long time before radiating light. A light source is called phosphorescent if the delay between energy absorption and emission is longer than one second. Phosphorescent materials can glow in the dark for several minutes after they have been exposed to strong light.

The aurora borealis and aurora australis (northern and southern lights) in the night sky in high latitudes are luminescent sources. Electrons in the solar wind that sweeps out from the sun become deflected in the earth's magnetic field and dip into the upper atmosphere near the north and south magnetic poles. The electrons then collide with atmospheric molecules, exciting the molecules' electrons and making them emit light in the sky.

Chemiluminescence occurs when a chemical reaction produces molecules with electrons in excited energy levels that can then radiate light. The color of the light depends on the chemical reaction. When chemiluminescence occurs in plants or animals it is called bioluminescence. Many creatures, from bacteria to fish, make light this way by manufacturing substances called luciferase and luciferin. Luciferase helps luciferin combine with oxygen, and the resulting reaction creates excited molecules that emit light. Fireflies use flashes of light to attract mates, and some fish use bioluminescence to attract prey, or confuse predators.

Synchrotron Radiation: Not all light comes from atoms. In a synchrotron light source, electrons are accelerated by microwaves and kept in a circular orbit by large magnets. The whole machine, called a synchrotron, resembles a large artificial atom. The circulating electrons can be made to radiate very monochromatic light at a wide range of frequencies.

Lasers: A laser is a special kind of light source that produces very regular waves that permit the light to be very tightly focused. Laser is actually an acronym for Light Amplification by Stimulated Emission of Radiation. Each radiating charge in a non-laser light source produces a light wave that may be a little different from the waves produced by the other charges. Laser sources have atoms whose electrons radiate all in step, or synchronously. As a result, the electrons produce light that is polarized, monochromatic, and coherent, which means that its waves remain in step, with their peaks and troughs coinciding, over long distances.

This coherence is made possible by the phenomenon of stimulated emission. If an atom is immersed in a light wave with a frequency, polarization, and direction the same as light that the atom could emit, then the radiation already present stimulates the atom to emit more of the same, rather than emit a slightly different wave. So the existing light is amplified by the addition of one more photon from the atom. A luminescent light source can provide the initial amplification, and mirrors are used to continue the amplification.

Lasers have many applications in medicine, scientific research, military technology, and communications. They provide a very focused, powerful, and controllable energy source that can be used to perform delicate tasks. Laser light can be used to drill holes in diamonds and to make microelectronic components. The precision of lasers helps doctors perform surgery without damaging the surrounding tissue. Lasers are useful for space communications because laser light can carry a great deal of information and travel long distances without losing signal strength.

Nature of Light
Light Emission | Electromagnetic Waves | Photons | Light Sources | Light Detection