Man's physical universe

xanabras

390 ENERGY MAY BE PROPAGATED BY VIBRATIONS

the number of stones thrown per minute. If the wave front moves out

twenty feet in one second and ten stones are thrown per second, there

will be ten waves produced, each two feet in length. The length of the

wave times the frequency (i.e., the number of waves per second) will

equal the velocity of the wave, or the distance covered by the wave

front in a second.

The Wavelike Nature of Light Is Confirmed by the Nicety with Which

It Fits into the Electromagnetic Spectrum.

When it is not in our power to discern what is ultimately true, we should

subscribe to what is most probable. — Descartes.

Clerk Maxwell (1831-1879) worked out equations which summarized

the knowledge of his time concerning electricity and magnetism, that

included the concept of ether and seemed to apply to light. Michael

Faraday had already suggested that light and electromagnetic forces

might be phenomena of ether. On the basis of these equations Maxwell

advanced the hypothesis that light is an electromagnetic wave. In

ISSS Heinrich Hertz showed that electromagnetic waves sent out from

an electric spark have all the properties of light rays and thus verified

Maxwell's hypothesis.

All radiant waves are now considered to be produced by the motion

of electromagnetic lines of force, and hence the radiation spectrum is

properly called the "electromagnetic spectrum." Electromagnetic

lines of force will be discussed in Unit VII.

Although the nature of light is not well understood, it fits into the

electromagnetic spectrum so beautifully that there is little doubt concerning

the wavelike nature of light. The wave length of any type of

electromagnetic radiation can be measured, and the electromagnetic

spectrum was constructed on the basis of such measurements. The

accompanying insert shows the electromagnetic spectrum as it is known

today.

Our knowledge of radiant energy is still very limited, and many

portions of the electromagnetic spectrum remain unexplored.

Ultraviolet Radiations Are of Great Importance.

The biological effects and chemical effects of light become conspicuous

in the shorter wave lengths just beyond the limit of visibility

to the eye, which fails to respond to any wave lengths shorter than

3800 angstroms. The atmosphere on a clear, dry day transmits light

of wave lengths as low as 2900 angstroms. In higher altitudes the

ultraviolet light is transmitted more than in lower altitudes, so that

a sunburn is rapidly obtained even on cold days at high altitudes.

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