Man's physical universe

xanabras

UNIT VI

LIGHT IS

SECTION 1

A FORM OF RADIANT ENERGY

Introduction.

Few people ever become curious about the nature of light, but

those who have become interested in the "why and how" of it have

been richly repaid. It is now known that light is a form of radiant

energy. Sometimes the term "light" is used in referring to all forms

of radiant energy, but in a narrower sense it refers only to that portion

of the electromagnetic spectrum of radiant energy to which the human

eye is sensitive.

of radiation.

The human eye sees but one octave of this vast scale

This visible octave is called light.

Light Is Wavelike in Nature.

The wavelike nature of radiant energy is easy to understand in this

age of the radio. Nearly everyone knows that broadcasting stations

set up disturbances which travel as waves whose lengths depend upon

the transmitting apparatus. The sun may be considered to be a lightwave

transmitter, and our eyes may be considered to be the receiving

sets. The chief difference between radio waves and light waves is their

frequency or the closeness with which they follow one another.

A study of water waves will help to illustrate the behavior of light.

The water waves created in a shallow pan of water may be made

visible by observing light reflected from the surface of the water onto

a screen.

If the pan is jarred, waves will be observed to start on opposite

sides of the pan and pass through each other at the middle.

When

they reach the sides opposite to those from which they started, they

will be observed to recoil.

This simple experiment illustrates two fundamental properties of

wave motion : (1) two sets of waves can pass through each other without

being altered; (2) waves can be reflected at the same angle that they

are received.

Another property of light was probably observed in the second

century by the Alexandrian, Ptolemy, who studied the refraction or

bending of light rays as they passed from one medium to another.

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