LIGHT.
Light is emitted from a luminous body. A luminous body is one in which
all of the particles are conceived to be in violent motion, which motion
is transmitted to a supposed ether. The existence of such ether cannot
be demonstrated, but it is supposed to exist because it is impossible to
think of anything being transmitted except through some medium. This
ether is a rather imponderable substance; it is supposed to pervade all
space; and it exists in all matter and in every vacuum. It is supposed
to be elastic without weight and capable of transmitting motion without
loss of energy or friction. It is, however, assumed to be modified to
some extent by the matter in connection with which it exists. Thus the
rate of transmission of light waves is different for air, glass, water,
and other substances through which they may pass and such bodies as are
entirely opaque are supposed to suppress the ether vibrations, resulting
in light entirely.
Light is then a mode of motion of this universal ether which pervades
all space even to the farthest star. The motion of this ether is
conceived to be about as illustrated in Figure 53. If we take a heavy
string or a small rope and, stretching it reasonably taut, jerk it
forward and back quickly a few times, it will be seen to move and assume
the appearance shown in the figure; and that portion shown between the
two vertical lines represents one complete wave.
[Illustration: FIGURE 53.]
The assumption of this ether and the vibratory motions of it form the
only explanation that is capable of accounting for all of the phenomena
of light. All other theories advanced have failed to stand the test;
sooner or later some phenomena have appeared which could not be
explained by them.
Light waves are known to travel through space in straight lines until
they meet with some medium which is capable either of reflecting,
refracting, or absorbing them. The rectilinear propagation of light,
consisting of vibratory motion, is one of the most difficult parts of
the theory to explain. It involves rather deep study and more
mathematics than the scope of this work will warrant. Suffice it to say
that the rectilinear propagation is brought about through the
interference of light waves. An analogy of this can be found in a stream
of water. It is well known that a stream of water issuing from the
nozzle of a garden hose moves in a straight line until gradually forced
into a spray by the resistance of the air. Yet the water passing through
the hose is interfered with on all sides and on all sides there is a
tendency to deflect it. If the water were to move along slowly so that
we could observe the action on one side independently of the balance of
the stream, we should see a series of waves being formed by every
particle of the hose which offers resistance to the flow. The waves
formed in the interior of the hose on all sides are interfered with by
waves from all other directions and the result is motion in a straight
line. In a similar way it may be conceived that the millions of light
waves emitted from a luminous body interfere with each other and thus
cause light rays to move in a straight line.
Light is a form of energy and can be converted into other forms of
energy. It can be converted into heat, for instance, or can be used to
produce chemical effects. Light differs from heat only in the rate of
vibration and the length of the ether waves. Heat can be reflected in
the same manner as light, as the following experiment will show: Arrange
two ordinary reflectors as shown in Figure 54. In the focal point of
one, place a heated iron ball or something of the kind; if this be hot
enough, it will ignite paper suspended in the focal point of the other
reflector, although a thermometer placed anywhere between the two
reflectors will give only a small indication of a rise in temperature.
[Illustration: FIGURE 54.]
All light rays as well as heat rays are in themselves invisible; we can
see only the object which emits or reflects them. If a beam of light be
allowed to enter a darkened room, as shown in Figure 55, we shall
probably be able to see the whole path of the rays illuminated, as well
as the spot on the floor. But this will be because of particles of dust
in the air which reflect the rays to our eyes. If we introduce some
smoke into the room, we shall see the light much more plainly because
there are now more particles of matter to reflect it. On the other hand,
if special precautions are taken to have the air absolutely clear of
dust, we shall be able to see nothing but the spot on the floor.
[Illustration: FIGURE 55.]
Light travels through space at the rate of about 186,000 miles per
second. White light is a combination of light of many colors, but the
speed of transmission is the same for all colors. The length of waves
and the rate of vibrations, however, vary. The red rays have the longest
waves and the slowest rate of vibration; they vibrate about three
hundred ninety-five billion times per second and the wave length is
about 0.0008 millimeters. The violet rays possess a wave length of about
seven hundred sixty-three billion per second. There are light waves
which are longer than the red rays and these are known as _infra red_.
They are not visible to the eye but their existence can be proved in
many ways. Light waves shorter than the violet are also invisible and
are known as _ultra violet_. These have much importance in photography
and to this class belong the X ray.
[Illustration: FIGURE 56.]
We have mentioned above that white light is a combination of light rays
of many colors. This can be proved by the following experiments: If we
arrange to have a beam of sunlight pass through a small hole into a
darkened room, it will pass to the wall on the opposite side in a
straight line and give us white illumination upon a small spot. If we
now arrange a prism in the path of this ray or beam of light, we shall
find that the light no longer passes straight to the wall but that
instead, it is bent in a certain direction and furthermore shows us a
brilliant array of colors. This is illustrated in Figure 56. The rays
are thus shown to be separated into their constituent colors; red is
shown at the top and the following colors merge imperceptibly into one
another--orange, yellow, green, blue, indigo, and finally violet at the
bottom.
The reason for this change is that the rays of light on entering the
glass are slowed down--those of the higher rate of vibration more than
the others. The violet rays are thus said to be more refrangible than
the red, for instance. The colors thus produced are simple colors. This
is proved by the fact that if the light is passed on through another
prism, it will be again reflected but will not be resolved into other
colors; although whichever color is carried to the next prism will
spread out and show finer gradations in its color.
[Illustration: FIGURE 57.]
The colors given above are those obtainable from the decomposition of
sunlight and make up what is known as the _solar spectrum_. If instead
of sunlight some other illuminant be used, the arrangement of colors
will be different; and it has been found possible to tell from the
colors of the spectrum what substances are burning, or heated to a
luminous degree, in the source from which the light comes. This method
is known as _spectrum analysis_.
There are several ways in which light, which has thus been separated
into its fundamental colors, can be re-composed so as to give us white
light again. One of these methods consists in arranging an inverted
prism to receive the light, as shown in Figure 57. The rays leave the
second prism parallel and produce the effect of white light. Another
method consists in gathering the rays from the prism by a lens, as
shown in Figure 58. Furthermore, if we take a disc and paint the colors
of the solar spectrum upon it in the proper proportions, as indicated in
Figure 59, and cause this disc to be rapidly revolved, we shall see it
as almost white.
[Illustration: FIGURE 58.]
[Illustration: FIGURE 59.]
Another fact which goes to prove the undulatory, or motion, theory of
light is that two sources of light arranged to oppose each other can
actually be made to produce darkness. To do this, the waves of one
source of light must be made so that they exactly oppose those of the
other; thus they destroy each other and destroy what light there is in
either. There are other methods, but this can be partially accomplished
in the following manner: Two small mirrors of black glass or of metal
are placed, as shown in Figure 60, very close together and so that they
form an angle of nearly 180 degrees. A beam of light arranged to fall
upon both of the mirrors will be reflected in such a manner that the two
halves interfere with each other and cause bands of light and darkness
to appear. The dark lines are due to the opposition and nullification of
certain of the light waves.
[Illustration: FIGURE 60.]
[Illustration: FIGURE 61.]
The intensity of light diminishes directly as the square of the distance
through which it is transmitted. This is illustrated in Figure 61. The
light, starting from a point, is limited by the size of the first square
at the left; it spreads out more and more, and illuminates larger and
larger spaces. Exact measurement will show that the spaces illuminated
by a ray of light are always exactly proportional to the square of the
distance from the point of light. This law, however, applies strictly
only if the distances considered are long compared to the source of
light, so that the light may be considered as being a mathematical
point, that is, having no physical dimensions. If the source of light,
for instance, were of the same size as the first opening and of uniform
intensity there would result the same intensity of illumination of a
similar space at all distances. There would, however, be an outer fringe
of light which would be proportional to the law of inverse squares. Many
reflectors are arranged to throw very nearly parallel rays; and with
these the intensity remains the same except for absorption, which is
ordinarily not very great.
We see things only through the rays of light they reflect. All colored
bodies have this peculiarity, viz., that they are capable of reflecting
only such rays as make up the color the body is said to possess. A red
body, for instance, absorbs all colors except red and reflects red only.
A black body absorbs all rays and a perfectly dull black body is visible
only by contrast; that is, we do not see it but we are aware that there
is something invisible before our eyes. When we are in a perfectly dark
room, we see nothing but we have blackness before our eyes. A perfectly
white body is one which reflects all of the rays of light and absorbs
none.
When we view things through colored glasses, we see them only in the
colors which the glass will transmit. If we view a red body through a
green glass or under a green light, it will appear black because it is
capable of reflecting only red rays and in the green light there are no
red rays; hence there is nothing to be reflected and the red appears
black.