David DeFario, age 14, of Columbus, Ohio, for his question:

HOW DOES A MERCURY VAPOR LAMP WORK:

A mercury vapor lamp is a type of electric light.

Mercury vapor lamps have two bulbs, one inside the other. The inner bulb, made of quartz, is called the arc tube. The outer bulb protects the arc tube, which contains mercury vapor at a higher pressure than that of a fluorescent lamp. Because of the high pressure, the lamp does not need a phosphor coating to produce visible light.

The mercury vapor gives off greenish blue light as well as ultraviolet rays. But the lamp produces almost no red light, and so red objects appear brown.

Lighting engineers use mercury vapor lamps in places where the deficiency of red light has little importance, such as in parking lots and along highways.

Mercury vapor lamps have a longer life than other electric lightsof similar brightness. But they do not produce full brightness immediately after being turned on. They take from three to seven minutes to build up the mercury vapor pressure and reach full brightness.