Man's physical universe

xanabras

COLORS ARE PORTIONS OF VISIBLE SPECTRUM 417

soap bubbles, tempered steel, and oil films on water produce colors in

the same way. Interference in light corresponds to interference in

sound, which is discussed on p.

46L

Colors May Also Be Produced by Scattering.

The colors of the sky are due to the scattering of light by molecules

of the air and particles floating in the air, just as the waves of the sea

are turned aside and scattered by rocks that rise above the surface.

The short waves of light that compose the blue end of the spectrum

are more easily turned aside than the longer, red waves, just as ripples

are turned aside by a rock over which the larger waves heave themselves

and steadily advance. Thus a separation takes place and color

is produced.

For example, the smoke rising from a chimney looks blue against a

dark background, especially if the smoke particles are quite small. If,

on the other hand, the smoke is viewed against a bright background of

luminous cloud or even against the sun itself, the color that comes

through is brown or red.

A red light carries better than a white light in a misty atmosphere

because the water particles do not scatter the long, red rays so much

as they scatter blue rays. Hence infrared photography is valuable in a

hazy atmosphere.

The setting sun and the rising full moon appear to be red because

at the horizon the light has to travel a much greater distance through

the atmosphere to reach the eye than it does when the sun or moon is

directly overhead. The shorter blue waves are scattered, and only the

longer rays toward the red end of the spectrum penetrate the great

thickness of the atmosphere.

The color of the sky in the morning and evening is largely due to

scattering of light by dust and water vapor. When Krakatoa exploded

and ejected huge quantities of very fine dust into the air half a centur>'

ago, the colors of sunsets all

over the world were strangely beautiful

for many months afterward, because the dust drifted around the world

and settled very slowly.

Cloud particles reflect all colors of the spectrum, and therefore

reflections from the clouds show masses of shining white; but when

clouds lie between us and the sun, they may intercept nearly all of the

light and appear black.

Much of the blue color of the sea is due to the reflection from the

sky. Under a leaden sky the sea looks gray. Near the shore the water

is green because the fine sand in suspension reflects yellow, which is

added to the blue light reflected from the sky.

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