Man's physical universe

xanabras

402 ENERGY MAY BE PROPAGATED BY VIBRATIONS

Special incandescent lamps, made of glass that permits the passage

of ultraviolet rays but absorbs the visible light, are also available. The

life of these lamps is short because of the high temperatures required

for ultraviolet production with filament sources.

More intense sources of ultraviolet rays use either an electric arc or a

mercury-vapor lamp with a suitable filter.

Modern photofiood lights produce radiations so rich in ultraviolet

rays that several of these lights, placed in a box with a window made

of a kind of glass that filters out all but the ultraviolet radiations, provide

an inexpensive source of ultraviolet rays.

The Wave Lengths of Ultraviolet Light Sources Vary.

The wave lengths of ultraviolet radiations may range from 136 angstroms

to 4000 angstroms.

The wave lengths of ultraviolet rays may be determined by chemical

reactions which are influenced by i certain range of wave lengths only.

Fig. 175. Germicidal lamps used to sterilize glasses in a cafeteria. (Courtesy of

the Westinghouse Electric and Manufacturing Company.)

Thus the range 2200-3200 A. liberates iodine from potassium iodide,

potassium nitrate is reduced to potassium nitrite by a range of 2200-

4000 A., and methylene blue is decolorized by light of 2500-4000 A.

The wave length most effective against bacteria is about 2600 A.

Radiations between 3300 and 4000 A. do not affect the body and do not

have germicidal action, while the shorter wave lengths produce "sunburns"

and conjunctivitis of the eyeball. The wave lengths emitted

by different ultraviolet-ray sources depend upon the temperature of

the radiating sources, the nature of the glass in the lamp, the nature

of the filter, and other factors.

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