What makes the ocean look green in some places and blue in
others?
What makes the sky blue?
What causes material to be colored?
What makes a rainbow?
What is color?
[Illustration: FIG. 90. Making a rainbow on the wall.]
Color is merely a kind of light. We say that a sweater is red; really
the sweater is not red, but the light that it reflects to our eyes is
red. We speak of a piece of red glass, but the glass is not red; it is
the light that it lets pass through it that is red.
White is not really a color; _all_ colors put together make white.
Experiments 50 and 51 will prove this.
[Illustration: FIG. 91. The prism separates the white light into the
rainbow colors.]
EXPERIMENT 50. Hold a prism in the sunlight by the window
and make a "rainbow" on the wall. The diagram here shown
illustrates how the prism breaks up the single beam of white
light into different-colored beams of light.
[Illustration: FIG. 92. When the wheel is rapidly whirled the colors
blend to make white.]
EXPERIMENT 51. Rotate the color disk on the rotator and watch
it. Make it go faster and faster until all the colors are
perfectly merged. What color do you get by combining all
the colors of the rainbow? If the colors on the disk were
perfectly clear rainbow colors, in exactly the same proportion
as in the rainbow, the whirling would give a white of dazzling
purity.
Since you can break up pure white light into all the colors, and since
you can combine all the colors and get pure white light, it is clear
that white light is made up of all the colors.
* * * * *
As we have already said, light is probably vibrations or waves of
ether. Light made of the longest waves that we can see is red. If the
waves are a little shorter, the light is orange; if they are shorter
yet, it is yellow; still shorter, green; shorter still, blue; while
the shortest waves that we can see are those of violet light. Black
is not a color at all; it is the absence of light. We say the night
is black when we cannot see anything. A deep hole looks black because
practically no light is reflected up from its depths. When you "see"
anything black, you really see the things around it and the parts of
it that are not perfectly black. A pair of shoes, for instance, has
particles of gray dust on them; or if they are very shiny they reflect
part of the light that strikes them as a white high-light. But the
really black part of your shoes would be invisible against an equally
black background.
A black thing absorbs the light that strikes it and turns it to heat.
Here is an experiment that will prove this to you:
EXPERIMENT 52. (a) On a sunny day, take three bottles, all of
the same size and shape, and pour water out of a pitcher or
pan into each bottle. Do not run the water directly from the
faucet into the bottle, because sometimes that which comes
out of the faucet first is warmer or colder than that which
follows; in the pitcher or pan it will all be mixed together,
and so you can be sure that the water in all three bottles is
of the same temperature to begin with. Wrap a piece of white
cotton cloth twice around one bottle; a piece of red or
green cotton cloth of the same weight twice around the second
bottle, and a piece of black cotton cloth of the same weight
twice around the third bottle, fastening each with a rubber
band. Set all three bottles side by side in the sunlight, with
2 or 3 inches of space between them. Leave them for about an
hour. Now put a thermometer into each to see which is warmest
and which is least warm.
From which bottle has most of the light been reflected back
into the air by the cloth around it? Which cloth absorbed most
of the light and changed it into heat? Does the colored cloth
absorb more or less light than the white one? than the black
one?
[Illustration: FIG. 93. Which color is warmest in the sunlight?]
(b) On a sunny day when there is snow on the ground, spread
three pieces of cotton cloth, all of the same size and
thickness, one white, one red or green, and one black, on top
of the snow, where the sun shines on them. Watch them for a
time. Under which does the snow melt first?
The white cloth is white because it reflects _all_ colors back
at once. It therefore absorbs practically no light. But the
reason the black cloth looks black is that it reflects almost
none of the colors--it absorbs them all and changes them to
heat. The colored cloth reflects just the red or the green
light and absorbs the rest.
Maybe you will understand color better if it is explained in another
way. Suppose I throw balls of all colors to you, having trained you to
keep all the balls except the red ones. I throw you a blue ball; you
keep it. I throw a red ball; you throw it back. I throw a green ball;
you keep it. I throw a yellow ball; you keep it. I throw two balls at
once, yellow and red; you keep the yellow and throw back the red. I
throw a blue and yellow ball at the same time; you keep both balls.
Now suppose I change this a little. Instead of throwing balls, I shall
throw lights to you. You are trained always to throw red light back
to me and always to keep (absorb) all other kinds of light. I throw a
blue light; you keep it, and I get no light back. I throw a red light;
you throw it back to me. I throw a green light; you keep it, and I get
no light back. I throw a yellow light; you keep it, and I get no light
back. I throw two lights at the same time, yellow and red; you keep
the yellow and throw back only the red. But yellow and red together
make orange; so when I throw an orange light, you throw back the red
part of it and keep the yellow.
Now if we suppose that instead of throwing lights to _you_ I throw
them to molecules of dye which are "trained" to throw back the red
lights and keep all the other kinds (absorb them and change them to
heat), we can understand what the dye in a red sweater does. The dye
is not really trained, of course, but for a reason which we do not
entirely understand, some kinds of dye always throw back (reflect) any
red that is in the light that shines on them, but they keep all other
kinds of light, changing them to heat. Other dyes or coloring matter
always throw back any green that is in the light that shines on them,
keeping the other colors. Blue coloring matter throws back only the
blue part of the light, and so on through all the colors.
So if you throw a white light, which contains all the colors, on a
"red" sweater, the dye in the sweater picks out the red part of the
white light and throws that back to your eyes (reflects it to you) but
it keeps the rest of the colors of the white light, changing them to
heat; and since only the red part of the light is reflected to your
eyes, that is the only part of it that you can see; so the sweater
looks red. The "green" substance (chlorophyll) in grass acts in the
same way; only it throws the green part of the sunlight back to your
eyes, keeping the rest; so the part of the light that reaches you from
the grass is the green light, and the grass looks green.
Anything white, like a piece of paper, reflects all the light that
strikes it; so if all the colors (white light) strike it, all are
reflected to your eyes and the object looks white.
You have looked at people under the mercury-vapor lights in
photo-postal studios, have you not? The lights are long, inclined
tubes which glow with a greenish-violet light. No matter how good the
color of a person is in ordinary light, in that light it is ghastly.
[Illustration: FIG. 94. A mercury-vapor lamp.]
Go into the kitchen tonight, light a burner of the gas stove, turn out
the light and sprinkle salt on the blue gas flame. The flame will leap
up, yellow. Look at your hands, at some one's lips, at a piece of red
cloth, in this light. Does anything look red?
The reason why nothing looks pink or red in these two kinds of light
is this: The light given by glowing salt vapor or mercury vapor has no
red in it; if you tried to make a "rainbow" from it with a prism, you
would find no red or orange color in it. A thing looks red when it
absorbs all the parts of the light that are not red and reflects the
red light to your eyes. If there is no red in the light to reflect,
obviously a thing cannot look red in that light.
When you look through a piece of colored glass, the case is somewhat
different. A piece of blue glass, for instance, acts as a sort of
strainer. The coloring matter in it lets the blue light through it,
but it holds back (absorbs) the other kinds of light. So if you look
through a piece of blue glass you see everything blue; that is, only
the blue part of the light from different objects can reach your eyes
through this kind of glass. Anything that is transparent and colored
acts in a similar way.
WHY THE SKY IS BLUE. And that is why the sky looks blue. Air holds
back all colors of light except blue; that is, it holds them back a
little. A room full of air holds the colors back hardly at all. A
few miles of air hold them back more; mountains in the distance look
bluish because only the blue light from them can reach you through
the air. The hundred or more miles of air above you hold back a
considerable amount of the other colors of light, letting through much
more of blue than of any other color. So the sky looks blue; that is,
when the air scatters the sunlight above you, it is chiefly the blue
parts of the sunlight that it allows to reach your eyes.
WHY BODIES OF WATER LOOK GREEN IN SOME PLACES AND BLUE IN OTHERS.
Water acts in a similar way, but it lets the green light through
instead of the blue. A little water holds back (absorbs) the other
colors so slightly that you cannot notice the effect in a glass of
water. But in a swimming tank full of water, or in a lake or an ocean,
you can notice it decidedly when you look straight down into the water
itself.
When you look at a smooth body of water at a slant on a clear day,
the blue sky is reflected to you and the water looks blue instead of
green. And it may even look blue when you look straight down in it if
it is too deep for you to see the bottom and the sky is reflected from
the surface.
WHY THE SKY IS OFTEN RED AT SUNSET. Dust lets more of red and yellow
light through than of any other color, and for this reason there is
much red and yellow in the sunset. Just before the sun sets, it shines
through the low, dusty air. The dust filters out most of the light
except the red and yellow. The red light and yellow light are
reflected by the clouds (for the clouds are themselves "white"; that
is, they reflect all the colors that strike them), and you have the
beautiful sunset clouds. Sometimes there is a purple in the sunset,
and even green. But since the air itself is blue (that is, it lets
mostly blue light go through), it is easy to see how this blue can
combine with the red or yellow that the dust lets through, to form
purple or green.
But we could not have sunset colors or all the colors we see on earth,
if it were not that the sunlight is mostly white--that it contains all
colors. And that, too, is why we can have a rainbow.
HOW RAINBOWS ARE FORMED. You already know fairly well how a rainbow
is formed, since you made an imitation of one with a prism. A rainbow
appears in the sky when the sun shines through the rain; the plain
white light of the sun is divided up into red, orange, yellow, green,
blue, indigo, and violet. As the white light of the sun passes through
the raindrops, the violet part of the light is bent more than any of
the rest, the indigo part is bent not quite so much, and so on to the
red, which is bent least of all. So all the colors fan out from the
single beam of white light and form a band of color, which we call the
rainbow.
[Illustration: FIG. 95. Explain why the breakers are white and the sea
green or blue.]
HOW WE CAN TELL WHAT THE SUN AND STARS ARE MADE OF. When a gas or
vapor becomes hot enough to give off light (when it is incandescent),
it does not give off white light but light of different colors. An
experiment will let you see this for yourself.
EXPERIMENT 53. Sprinkle a little copper sulfate (bluestone)
in the flame of a Bunsen burner. What color does it make the
flame?
Copper vapor always gives this greenish-blue light when it is heated.
The photographer's mercury-vapor light gave a greenish-violet glow.
When you burn salt or soda in a gas flame, you remember that you get a
clear yellow light. By breaking up these lights, somewhat as you broke
up the sunlight with the prism, chemists and astronomers can tell what
kind of gas is glowing. The instrument they use to break up the light
into its different colors is called a _spectroscope_, and the band
of colors formed is called the _spectrum_. With the spectroscope they
examine the light that comes from the sun and stars and by the colors
of the spectra they can tell what these far-distant bodies are made
of.
_APPLICATION 39._ If you were going to the tropics, would it
be better to wear outside clothes that were white or black?
_APPLICATION 40._ A dancer was to dance in a spotlight on the
stage. The light was to change colors constantly. She wanted
her robe to reflect each color that was thrown on it. Should
she have worn a robe of red, yellow, white, green, or blue?
_APPLICATION 41._ If you looked through a red glass at a
purple flower (purple is red mixed with blue), would the
flower look red, blue, purple, black, or white?
INFERENCE EXERCISE
Explain the following:
241. Mercury is separated from its ore by heating the ore so
strongly that the mercury rises from it as a vapor.
242. Hothouses are built of glass.
243. A "rainbow" is sometimes seen in the spray of a garden
hose.
244. Your feet become hot when your shoes are being polished.
245. Doors into offices usually have windows of ground glass
or frosted glass.
246. Opera glasses are of value to those sitting at a distance
from the stage.
247. In order to see clearly through opera glasses, you have
to adjust them.
248. It is warm inside an Eskimo's hut although it is built of
ice and snow.
249. It is usually cooler on a lawn than on dry ground.
250. Black clothes are warmer in the sunlight than clothes of
any other color.
CHAPTER SIX
SOUND