When an electron is farthest away from the nucleus of its atom, it has the most potential energy. An electron that is further away from the nucleus than another is in what we call a higher energy level. If an electron moves from a position close to the nucleus of the atom to a position farther from the nucleus, that is from a lower to a higher energy level, we say the atom (not just the electron) has been excited. The electron in an excited atom quickly moves back down to its original level. It is de-excited. As the electron moves down, it loses energy. It gives off this lost energy in the form of electromagnetic radiation. The atom emits light, due to the process of excitation and de-excitation.

Each emission of light energy from an atom occurs in a pulse of radiation called a photon. A photon can be thought of as a particle of pure energy. The energy of a photon is directly proportional to its frequency. (Yes, photons have frequencies even though they can be thought of as particles.) E~f. Different energy levels therefore confirm to different frequencies of light. Colored light (in a candle flame, for example) is therefore a result of the electrons in the excited atoms moving to different energy levels and emitting different frequencies of light. Each element has a different pattern of electron energy levels, and so each element emits light with a characteristic pattern of frequencies. This is the element's emission spectrum. The emission spectrum of an element can be recorded by a tool called a spectroscope. A spectroscope passes light through focusing lenses, a small slit, and a prism to separate the light into it's different frequencies. These are recorded as thin lines the width of the slit onto a photographic film. The brighter each color of the line, the more of that frequency is given off by that element. Since each element has only one emission spectrum, a spectroscope can be used to determine the chemical make up of distant light producing sources.

Incandescent light is light that is produced as a result of high temperature. Incandescent light differs from "normally" emitted light in that its emission spectrum is composed of an infinite number of frequencies. The reason incandescent light emits an infinite number of frequencies is that atoms undergoing incandescence are always packed close together. Consider several radio transmitters. If the radios are far apart, the sounds produced from each can be heard clearly and easily. If the radios are all put together in one box, though, the sound will be jumbled and generic. The same type of thing occurs with atoms packed close together. They all bounce off of and interfere with each other, producing an infinite number of frequencies. The properties of incandescent light are determined by the temperature of the source. The frequency most emitted by an incandescent substance is called the peak frequency. The peak frequency increases as temperature increases. Peak frequency is directly proportional to temperature. f(p)~T.

The emission spectrum from an incandescent light source is continuous. However, if we send incandescent light through a gas and then through a spectroscope, the spectrum will not be continuous. Instead, there will be several dark lines in it. This is an absorption spectrum, and it occurs because atoms absorb as well as emit light. An atom most strongly absorbs the same frequency of light as it emits. Therefore, the absorption lines in an incandescent spectrum correspond exactly to the emission lines that would be produced by the gas doing the absorbing. An interesting result of absorption spectrums occurred with the sun. The sun is an incandescent source, and so its spectrum should be continuous. However, the sun's spectrum has numerous absorption lines in it. This proves that the sun is surrounded by some gas. The absorption lines were discovered to be different from any element known at that time, and so a new element was discovered: Helium.

We know from the proportionality E~f that ultraviolet light has more energy than visible light. The atoms in some materials can be excited by ultraviolet light. Because of the great energy of ultraviolet light, the electrons may jump several energy levels instead of just one. When the atom de-excites, the electrons move down only one energy level at a time, producing visible light. This process of producing visible light when excited by ultraviolet light is called fluorescence. This is the basic process behind fluorescent lamps. In a fluorescent lamp, mercury vapor is excited by vibrating electrons. The mercury atoms produce ultraviolet light. This light strikes a material called phosphors in the tube. These phosphors then emit visible light in many different frequencies. These frequencies combine to produce white light.

Light emitted from a common source of light is incoherent. Photons of many frequencies in many phases of vibration are produced by them. Filtered light is also incoherent, because although it is composed of only one frequency, its vibrations are still out of phase. Coherent light has only one frequency and its vibrations are totally in phase.

A laser is a tool that produces coherent light. A laser only works when, for at least a brief period of time, there are more atoms in a higher energy level than a lower energy level in the laser. A common type of laser consists of an impure ruby crystal rod (filled with cronium) surrounded by a flash tube that sends high intensity green light into the rod. As electrons in the crystal de-excite, they release red light. Red photons hit neighboring atoms. Since most of the atoms are still in the excited state, their impact of the photon triggers emission of light of the exact same frequency (in this case red). Most of this light travels out of the crystal. Some of it, however, travels parallel to the crystal. The ends of the crystal are coated as a mirror. One end is totally reflecting, and the other semi-reflecting. The parallel moving light reflects off the mirror and runs into atoms. This triggers the emission of more light that moves parallel to the crystal. The light waves are built up, and eventually they escape through the semi-reflecting mirror in a laser pulse. This process occurs every time the flash is fired.

In addition to crystal lasers, there are also gas, chemical, class, and liquid lasers. Most of these lasers produce a continuous beam of laser light instead of pulses. Lasers are used everywhere these days. Laser surgery, laser communication, and laser coding are just a few of the uses of this amazing device. The power of light is just beginning to be tapped.