Man's physical universe



Roger Bacon (1210-1292) studied the general phenomena of refraction

and described the laws of reflection. In 1678 Christian Iluygens, a

Dutchman, developed the wave theory of light which was first suggested

by Leonardo da Vinci. A few years later Sir Isaac Newton

advanced a different theory. Newton's work on light and optics alone

was sufficient to place his name on the roster of the world's greatest

scientists. The Pythagoreans believed that light consisted of particles

projected into the eye. Newton likewise believed that light must

consist of a stream of corpuscles, for he could not account for sharp

shadows in terms of wave motion. Water waves bend around obstacles

and do not produce shadows. The same thing is true in the case of

sound waves, except that the shorter wave lengths of sound will produce

shadows. A hundred years after Newton it was shown that the

extreme smallness of the wave lengths of light compared with the

dimensions of the objects placed in their path explains the sharpness

of the shadows produced. Strangely enough, the colored rings produced

when light is passed through a polished glass plate in close

contact with a lens of small curvature, called "Newton's rings," are

now accepted as one of the best proofs of the wave theory of light.

It remained for Thomas Young (1773-1829) to revive Huygens' wave

theory of light, which had been rejected for so long, largely on account

of Newton's great prestige. In 1801 he concluded, as the result of

studies in interference, that light was propagated in the form of waves

rather than particles. Water waves show interference; thus, when two

waves meet so that the crest of one coincides with the trough of another,

they neutralize each other. When Young passed a narrow beam of

light through two narrow slits in a screen, the rays from the two holes

overlapped on a second screen, producing a series of brilliantly colored

bands. He reasoned that the light waves from one slit have to travel

farther than those from the other slit, so that, at points where the

crests of one wave coincide with the troughs of another, bands of darkness

are produced. At other points two crests would coincide to produce

light bands of double the intensity produced by the light from

either hole. When the light source was of a single wave length, the

bands were thus alternately dark and bright; but when white light was

used, the bands were colored, because white light consists of a mixture

of wave lengths.

If any one wave length is extracted from white light,

the mixture of wave lengths left produces a complementary ^ color.

Young measured the wave lengths of different colored lights in this

way; the dimensions of his apparatus and the breadth of the light bands

> Complementary colors are those colors which produce white light when mixed with

each other in the proper proportions.

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