Making Light

There are two basic types of light sources. Incandescence involves the vibration of entire atoms, while luminescence involves only the electrons.

Incandescent light is produced when atoms are heated and release some of their thermal vibration as electromagnetic radiation. It is the most common type of light that you see everyday sunlight, regular light bulbs (not florescent) and fires are all incandescent sources of light. Incandescent light is also known as "black body radiation." This seemingly self-contradictory name arises from the history of physics-scientists studying this type of light emission modeled their theories on ideal materials that would absorb all colors of light, hence appearing to be "black bodies". Depending on how hot the material is, the photons released have different energies, and therefore, different colors. It was found that at lower temperatures, these materials would emit radiation in the infrared wavelengths which we feel as heat (fires, for example, emit most of their energy in the infrared). As temperatures are increased, increasingly more energetic radiation is emitted, so these materials would glow red, then orange, then yellow, and eventually "white-hot." Although ideal black body materials don't exist in reality, most substances are close enough that this color sequence can be observed. This is why a fire tends to be redder than a halogen lamp-the filament in a halogen lamp is heated to a higher temperature than normal fires. Likewise, the hottest stars appear to be a blueish-white while cooler stars such as our sun are more yellowish in appearance. Some sources of incandescent light are: the sun, fire and light bulbs.

Unlike incandescence, luminescent light occurs at lower temperatures, because it is produced when an electron releases some of its energy to electromagnetic radiation, not an entire atom. It turns out that electrons like to have energy at specific "energy levels." Thus, when an electron jumps down to a lower energy level, it will release a specific amount of energy which becomes a photon, or light of a specific color. Therefore, continued luminescence requires something to continuously give the electrons a boost to a higher energy level to keep the cycle going. This boost may be provided by many sources: electrical current as in florescent lights, neon light, mercury-vapor street lights, light emitting diodes, television screens and computer monitors; chemical reactions as in Halloween light sticks and fire-flies; or radioactivity as in luminous paints, to name just a few examples.

Next article: Colors