Man's physical universe



yellow-green light. The great disadvantage of these lamps is that the

faces of people take on a deathlike appearance in their yellow-green

light, and colors cannot be matched. This difficulty is partly overcome

by the use of reflectors, on which red fluorescent materials have been

placed so as to add the red rays that are missing, and by locating

incandescent lamps in the same fixture.

A new type of mercury-vapor lamp used in television studios because

it gives such an intense, yet cool, light is the water-cooled mercuryvapor

quartz-tube light. The lamp itself is a quartz tube of very fine

bore about 13^ inches long, about the size of a cigarette. Tungsten

wires are sealed into each end ; and a small amount of mercury, together

with argon at a pressure of about one pound per square inch, is contained

in the tube. When the electricity is turned on, the mercury

vaporizes and produces a pressure of about 1200 pounds per square

inch. The tube produces a great deal of heat and must be cooled by

water. Its light output is 65,000 lumens, and it is K as bright as the

sun. Mercury-vapor lamps are of special interest because they are

excellent sources of ultraviolet radiations, which cause fluorescence.

Sodium-vapor Lamps Produce a Yellow Light That Is

Certain T3rpes of Illumination.

Excellent for

The sodium-vapor lamp is another type of lamp which employs

electroluminescence. The sodium-vapor lamp employs the principle

of the mercury-vapor lamps, except that sodium replaces the mercury.

The sodium-vapor lamp contains neon gas for starting and thus has

the characteristic red color of neon luminescence until the lamp gets

warm enough to vaporize the sodium.

Sodium-vapor lights are especially adapted for safety lighting for

highways, bridges, and underpasses, their chief advantage being their

relatively high efficiency as compared with tungsten-filament lamps.

Electromagnetic Waves Shorter than Those of Visible Light Produce

Fluorescence and Phosphorescence.

Some materials, when irradiated by light of one wave length (color),

will emit light of a different, but always longer, wave length. Such

radiations are called "luminescence," or "cold light." The explanation

is based on the electronic theory of the atomic structure, to be discussed

in the next Unit.

It is sufficient to state here that according to

the electron theory this light is emitted when electrons which have been

displaced from their normal or stable position in the atoms by bombardment

with some form of energy particles return to their original


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