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		Lightning strikes behind a farm house.
		Bridge and Lightning at Sydney Harbor.
		Bright neon beacons the weary in downtown Paris.
	Energy-level scheme of an atom of sodium, showing some of the allowed transitions in returning back to the ground state after ionization.

Some atoms will make colors when excited. In gas excitations it is specific atoms, present as a vapor or a gas, which have their electrons raised into higher energy levels by a variety of excitations; light is then emitted when the excess energy is released as photons. (This is in contrast to incandescence which can occur when any substance is heated.) Examples include electrical excitations as in arcs, sparks, lightning, neon tubes, and sodium and mercury-vapor lamps; chemical excitation as in the chemists' flame test for sodium and a few other elements (also utilized in pyrotechnic devices); and high-energy particles as in the northern and southern auroral displays. An unusual occurrence is found in triboluminescence, as when we crunch a wintergreen "lifesaver" in front of a mirror in the dark. The high-voltage field produced during the formation of new electrically-charged sugar crystal surfaces accelerates electrons which excite nitrogen-gas molecules to produce the ion N₂⁺ which has a blue luminescence; some ultraviolet is also produced and causes the oil of wintergreen (methyl salicylate) to fluoresce with a particularly intense blue-light production.

Above right, lightning dances across the charged skies over Sydney Harbour and the Harbour Bridge. The bridge is lit with vapor lamps, and the lightning is also an electrical excitation.

Sodium- and mercury-vapor lamps efficiently emit yellow and bright blue, respectively, and are often used in parking lots, where they distort the color of our autos. The mercury also produces much ultraviolet light; in fluorescent-tube lamps this is converted by a phosphor coating into lower-energy yellow, orange, and red light to produce a better color balance for indoor lighting. Gas lasers, as in the helium-neon laser, use gas excitations with optical feedback from mirrors at each end of the tube to produce coherent light, in which all the light waves have almost the same frequency and are coherent, that is, in step, both in space and in time.

Some of the possible energy levels for a gaseous sodium atom are shown in the diagram above. A high voltage, as in a sodium vapor lamp, produces ionization from the initial "ground state" into Na⁺ plus one electron at the top line of this figure. As the electron recombines the system passes along transitions, only a few of which are shown as arrows, with the emission of radiation. The position of the various levels shown, as well as the specific transitions which the system can follow, are explained by quantum theory; all paths are constrained by the "selection rules" to terminate in the lowest two arrows, which correspond to the emission of the bright-yellow sodium "doublet" lines at 589.7 and 589.0 nm.

Testing the inner tube of a mercury lamp at the Westinghouse Lamp Division, Bloomfield, N.J., in 1937.

A high intensity mercury vapor lamp is really a tube within a tube. The smaller tube contains Mercury vapor and is the source of light. In 1937, this was the type of lamp used to light working areas in the Douglas Aircraft Company plant in Long Beach, California.

Vapor lamps were a major advance because they are more energy efficient. The table below compares the mechanism and energy efficiency of various lamps. See a comparison of the colors produced by several of these lamps.

Bulb	type	Purpose	How it works	Efficiency (Lumens per Watt)
Regular incandescent bulb	filament bulb	Common household lightbulb	Contains about 20 in (50 cm) of coiled tungsten filament surrounded by inert gasses, such as argon, at low pressure.	4-20
Metal halide lamp	vapor lamp	common in large retail stores (like bookstores) where the colors must be accurate	Light is produced by an electric discharge through combined vapors of mercury and other metals. These metals are introduced into the arc tube as compounds of iodine. The compounds have the effect of increasing the light output in the blue, green and yellow-colored bands of the visual spectrum, resulting in a color appearance that is nearly pure white.	80-125
Mercury vapor lamp	vapor lamp	First high intensity discharge lamps, but now mostly obsolete.	Electric discharge through a vapor of mercury	32-63
Low-pressure sodium lamp	vapor lamp	sometimes used for street lighting	It contains a small amount of sodium, which takes a few minutes to vaporize when the lamp is switched on	up to 200
High-pressure sodium lamp	vapor lamp	Street lighting in cities, sports arenas	Contain sodium and aluminum, which combine to make pinkish-blue light. These lamps are quite efficient at converting electricity to light.	40-140

* Initial Efficiency (Lumens per Watt). Lumens-per-watt (or lamp efficacy) ratings reflect only the watts consumed by the lamp itself and, therefore, do not include the power consumption of the ballast, known as ballast loss.

 

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