Man's physical universe



In these cases the term "ray" refers to straight-line propulsion

rather than to wavelike radiations.

Infrared Rays Are Heat Rays.

Infrared rays, or heat rays, are invisible radiations contained in

sunlight which are longer in wave length than the visible light rays.

Everyone is familiar with the fact that heat can be radiated. The use

of reflectors in

electric and gas heating devices shows that heat rays

can be reflected just as visible rays are reflected.

Smokestacks or airplane exhausts may be detected by the effect of

their infrared radiations on sensitive thermocouples six miles away, and

icebergs may be located in the fog by their coldness.

A "fog-eye" recently developed consists of a reflector which gathers

in infrared rays and focuses them on a thermocouple. The electricity

generated by the thermocouple is amplified millions of times by

amplifiers and may be used to ring warning gongs when a ship

is approaching an iceberg. The "fog-eye" can detect a difference of

temperature of one fifty-thousandth of a degree, which means that

it should respond to the heat of a candle eight miles away. This instrument

is mounted on a tripod, which is kept swinging back and

forth through a semicircle, scanning the horizon through fog, smoke,

or the blackness of night. Had the Titanic been equipped with a

"fog-eye," one of the greatest marine disasters of history would have

been averted.

Thermocouples thus do for infrared radiations what the photoelectric

cell does for visible and ultraviolet radiations. A thermoelectric

sextant has been devised to measure the position of the sun through

obscuring clouds by means of infrared radiations.

Inasmuch as infrared rays penetrate the atmosphere better than the

radiations of shorter wave length and inasmuch as they affect certain

photographic emulsions, infrared rays may be used in photography in

cases where the shorter waves are scattered by the atmosphere.


photographs are often made of distant objects on hazy days by use

of films especially sensitive to infrared rays and employing an infrared

filter that screens out all but the infrared rays.

Figure 169 shows what can be done with infrared photography.

Pictures have been taken in a dark room with the heat rays from a hot

iron, as shown in Figs. 170 and 171.

In infrared photographs, the leaves of trees and grass appear white.

Because no known green paints have the same appearance in infrared

photographs, it is possible to distinguish between natural foliage and


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