COLOR
126. The Rainbow. One of the most beautiful and well-known phenomena
in nature is the rainbow, and from time immemorial it has been
considered Jehovah's signal to mankind that the storm is over and that
the sunshine will remain. Practically everyone knows that a rainbow
can be seen only when the sun's rays shine upon a mist of tiny drops
of water. It is these tiny drops which by their refraction and their
scattering of light produce the rainbow in the heavens.
The exquisite tints of the rainbow can be seen if we look at an object
through a prism or chandelier crystal, and a very simple experiment
enables us to produce on the wall of a room the exact colors of the
rainbow in all their beauty.
[Illustration: FIG. 86.--White light is a mixture of lights of rainbow
colors.]
127. How to produce Rainbow Colors. _The Spectrum._ If a beam of
sunlight is admitted into a dark room through a narrow opening in the
shade, and is allowed to fall upon a prism, as shown in Figure 86, a
beautiful band of colors will appear on the opposite wall of the room.
The ray of light which entered the room as ordinary sunlight has not
only been refracted and bent from its straight path, but it has been
spread out into a band of colors similar to those of the rainbow.
Whenever light passes through a prism or lens, it is dispersed or
separated into all the colors which it contains, and a band of colors
produced in this way is called a spectrum. If we examine such a
spectrum we find the following colors in order, each color
imperceptibly fading into the next: violet, indigo, blue, green,
yellow, orange, red.
128. Sunlight or White Light. White light or sunlight can be
dispersed or separated into the primary colors or rainbow hues, as
shown in the preceding Section. What seems even more wonderful is that
these spectral colors can be recombined so as to make white light.
If a prism _B_ (Fig. 87) exactly similar to _A_ in every way is placed
behind _A_ in a reversed position, it will undo the dispersion of _A_,
bending upward the seven different beams in such a way that they
emerge together and produce a white spot on the screen. Thus we see,
from two simple experiments, that all the colors of the rainbow may be
obtained from white light, and that these colors may be in turn
recombined to produce white light.
[Illustration: FIG. 87.--Rainbow colors recombined to form white
light.]
White light is not a simple light, but is composed of all the colors
which appear in the rainbow.
129. Color. If a piece of red glass is held in the path of the
colored beam of light formed as in Section 127, all the colors on the
wall will disappear except the red, and instead of a beautiful
spectrum of all colors there will be seen the red color alone. The red
glass does not allow the passage through it of any light except red
light; all other colors are absorbed by the red glass and do not reach
the eye. Only the red ray passes through the red glass, reaches the
eye, and produces a sensation of color.
If a piece of blue glass is substituted for the red glass, the blue
band remains on the wall, while all the other colors disappear. If
both blue and red pieces of glass are held in the path of the beam, so
that the light must pass through first one and then the other, the
entire spectrum disappears and no color remains. The blue glass
absorbs the various rays with the exception of the blue ones, and the
red glass will not allow these blue rays to pass through it; hence no
light is allowed passage to the eye.
An emerald looks green because it freely transmits green, but absorbs
the other colors of which ordinary daylight is composed. A diamond
appears white because it allows the passage through it of all the
various rays; this is likewise true of water and window panes.
Stained-glass windows owe their charm and beauty to the presence in
the glass of various dyes and pigments which absorb in different
amounts some colors from white light and transmit others. These
pigments or dyes are added to the glass while it is in the molten
state, and the beauty of a stained-glass window depends largely upon
the richness and the delicacy of the pigments used.
130. Reflected Light. _Opaque Objects._ In Section 106 we learned
that most objects are visible to us because of the light diffusely
reflected from them. A white object, such as a sheet of paper, a
whitewashed fence, or a table cloth, absorbs little of the light which
falls upon it, but reflects nearly all, thus producing the sensation
of white. A red carpet absorbs the light rays incident upon it except
the red rays, and these it reflects to the eye.
Any substance or object which reflects none of the rays which fall
upon it, but absorbs all, appears black; no rays reach the eye, and
there is an absence of any color sensation. Coal and tar and soot are
good illustrations of objects which absorb all the light which falls
upon them.
131. How and Why Colors Change. _Matching Colors._ Most women prefer
to shop in the morning and early afternoon when the sunlight
illuminates shops and factories, and when gas and electricity do not
throw their spell over colors. Practically all people know that
ribbons and ties, trimmings and dresses, frequently look different at
night from what they do in the daytime. It is not safe to match colors
by artificial light; cloth which looks red by night may be almost
purple by day. Indeed, the color of an object depends upon the color
of the light which falls upon it. Strange sights are seen on the
Fourth of July when variously colored fireworks are blazing. The child
with a white blouse appears first red, then blue, then green,
according as his powders burn red, blue, or green. The face of the
child changes from its normal healthy hue to a brilliant red and then
to ghastly shades.
Suppose, for example, that a white hat is held at the red end of the
spectrum or in any red light. The characteristics of white objects is
their ability to reflect _all_ the various rays that fall upon them.
Here, however, the only light which falls upon the white hat is red
light, hence the only light which the hat has to reflect is red light
and the hat consequently appears red. Similarly, if a white hat is
placed in a blue light, it will reflect all the light which falls upon
it, namely, blue light, and will appear blue. If a red hat is held in
a red light, it is seen in its proper color. If a red hat is held in a
blue light, it appears black; it cannot reflect any of the blue light
because that is all absorbed and there is no red light to reflect.
A child wearing a green frock on Independence Day seems at night to be
wearing a black frock, if standing near powders burning with red,
blue, or violet light.
132. Pure, Simple Colors--Things as they Seem. To the eye white
light appears a simple, single color. It reveals its compound nature
to us only when passed through a prism, when it shows itself to be
compounded of an infinite number of colors which Sir Isaac Newton
grouped in seven divisions: violet, indigo, blue, green, yellow,
orange, and red.
We naturally ask ourselves whether these colors which compose white
light are themselves in turn compound? To answer that question, let us
very carefully insert a second prism in the path of the rays which
issue from the first prism, carefully barring out the remaining six
kinds of rays. If the red light is compound, it will be broken up into
its constituent parts and will form a typical spectrum of its own,
just as white light did after its passage through a prism. But the red
rays pass through the second prism, are refracted, and bent from this
course, and no new colors appear, no new spectrum is formed. Evidently
a ray of spectrum red is a simple color, not a compound color.
If a similar experiment is made with the remaining spectrum rays, the
result is always the same: the individual spectrum colors remain
simple, pure colors. _The individual spectrum colors are groups of
simple, pure colors._
[Illustration: FIG. 88.--Violet and green give blue. Green, blue, and
red give white.]
133. Colors not as they Seem--Compound Colors. If one half of a
cardboard disk (Fig. 88) is painted green, and the other half violet,
and the disk is slipped upon a toy top, and spun rapidly, the rotating
disk will appear blue; if red and green are used in the same way
instead of green and violet, the rotating disk will appear yellow. A
combination of red and yellow will give orange. The colors formed in
this way do not appear to the eye different from the spectrum colors,
but they are actually very different. The spectrum colors, as we saw
in the preceding Section, are pure, simple colors, while the colors
formed from the rotating disk are in reality compounded of several
totally different rays, although in appearance the resulting colors
are pure and simple.
If it were not that colors can be compounded, we should be limited in
hue and shade to the seven spectral colors; the wealth and beauty of
color in nature, art, and commerce would be unknown; the flowers with
their thousands of hues would have a poverty of color undreamed of;
art would lose its magenta, its lilac, its olive, its lavender, and
would have to work its wonders with the spectral colors alone. By
compounding various colors in different proportions, new colors can be
formed to give freshness and variety. If one third of the rotating
disk is painted blue, and the remainder white, the result is lavender;
if fifteen parts of white, four parts of red, and one part of blue are
arranged on the disk, the result is lilac. Olive is obtained from a
combination of two parts green, one part red, and one part black; and
the soft rich shades of brown are all due to different mixtures of
black, red, orange, or yellow.
134. The Essential Colors. Strange and unexpected facts await us at
every turn in science! If the rotating cardboard disk (Fig. 88) is
painted one third red, one third green, and one third blue, the
resulting color is white. While the mixture of the spectral colors
produces white, it is not necessary to have all of the spectral colors
in order to obtain white; because a mixture of the following colors
alone, red, green, and blue, will give white. Moreover, by the mixture
of these three colors in proper proportions, any color of the
spectrum, such as yellow or indigo or orange, may be obtained. The
three spectral colors, red, green, and blue, are called primary or
essential hues, because all known tints of color may be produced by
the careful blending of blue, green, and red in the proper
proportions; for example, purple is obtained by the blending of red
and blue, and orange by the blending of red and yellow.
135. Color Blindness. The nerve fibers of the eye which carry the
sensation of color to the brain are particularly sensitive to the
primary colors--red, green, blue. Indeed, all color sensations are
produced by the stimulation of three sets of nerves which are
sensitive to the primary colors. If one sees purple, it is because the
optic nerves sensitive to red and blue (purple equals red plus blue)
have carried their separate messages to the brain, and the blending of
the two distinct messages in the brain has given the sensation of
purple. If a red rose is seen, it is because the optic nerves
sensitive to red have been stimulated and have carried the message to
the brain.
A snowy field stimulates equally all three sets of optic nerves--the
red, the green, and the blue. Lavender, which is one part blue and
three parts white, would stimulate all three sets of nerves, but with
a maximum of stimulation for the blue. Equal stimulation of the three
sets would give the impression of white.
A color-blind person has some defect in one or more of the three sets
of nerves which carry the color message to the brain. Suppose the
nerve fibers responsible for carrying the red are totally defective.
If such a person views a yellow flower, he will see it as a green
flower. Yellow contains both red and green, and hence both the red and
green nerve fibers should be stimulated, but the red nerve fibers are
defective and do not respond, the green nerve fibers alone being
stimulated, and the brain therefore interprets green.
A well-known author gives an amusing incident of a dinner party, at
which the host offered stewed tomato for apple sauce. What color
nerves were defective in the case of the host?
In some employments color blindness in an employee would be fatal to
many lives. Engineers and pilots govern the direction and speed of
trains and boats largely by the colored signals which flash out in the
night's darkness or move in the day's bright light, and any mistake in
the reading of color signals would imperil the lives of travelers. For
this reason a rigid test in color is given to all persons seeking such
employment, and the ability to match ribbons and yarns of all ordinary
hues is an unvarying requirement for efficiency.