LIGHT
98. What Light Does for Us. Heat keeps us warm, cooks our food,
drives our engines, and in a thousand ways makes life comfortable and
pleasant, but what should we do without light? How many of us could be
happy even though warm and well fed if we were forced to live in the
dark where the sunbeams never flickered, where the shadows never stole
across the floor, and where the soft twilight could not tell us that
the day was done? Heat and light are the two most important physical
factors in life; we cannot say which is the more necessary, because in
the extreme cold or arctic regions man cannot live, and in the dark
places where the light never penetrates man sickens and dies. Both
heat and light are essential to life, and each has its own part to
play in the varied existence of man and plant and animal.
Light enables us to see the world around us, makes the beautiful
colors of the trees and flowers, enables us to read, is essential to
the taking of photographs, gives us our moving pictures and our magic
lanterns, produces the exquisite tints of stained-glass windows, and
brings us the joy of the rainbow. We do not always realize that light
is beneficial, because sometimes it fades our clothing and our
carpets, and burns our skin and makes it sore. But we shall see that
even these apparently harmful effects of light are in reality of great
value in man's constant battle against disease.
99. The Candle. Natural heat and light are furnished by the sun, but
the absence of the sun during the evening makes artificial light
necessary, and even during the day artificial light is needed in
buildings whose structure excludes the natural light of the sun.
Artificial light is furnished by electricity, by gas, by oil in lamps,
and in numerous other ways. Until modern times candles were the main
source of light, and indeed to-day the intensity, or power, of any
light is measured in candle power units, just as length is measured in
yards; for example, an average gas jet gives a 10 candle power light,
or is ten times as bright as a candle; an ordinary incandescent
electric light gives a 16 candle power light, or furnishes sixteen
times as much light as a candle. Very strong large oil lamps can at
times yield a light of 60 candle power, while the large arc lamps
which flash out on the street corners are said to furnish 1200 times
as much light as a single candle. Naturally all candles do not give
the same amount of light, nor are all candles alike in size. The
candles which decorate our tea tables are of wax, while those which
serve for general use are of paraffin and tallow.
[Illustration: FIG. 57.--A photograph at _a_ receives four times as
much light as when held at _b_.]
100. Fading Illumination. The farther we move from a light, the less
strong, or intense, is the illumination which reaches us; the light of
the street lamp on the corner fades and becomes dim before the middle
of the block is reached, so that we look eagerly for the next lamp.
The light diminishes in brightness much more rapidly than we realize,
as the following simple experiment will show. Let a single candle
(Fig. 57) serve as our light, and at a distance of one foot from the
candle place a photograph. In this position the photograph receives a
definite amount of light from the candle and has a certain brightness.
If now we place a similar photograph directly behind the first
photograph and at a distance of two feet from the candle, the second
photograph receives no light because the first one cuts off all the
light. If, however, the first photograph is removed, the light which
fell on it passes outward and spreads itself over a larger area, until
at the distance of the second photograph the light spreads itself over
four times as large an area as formerly. At this distance, then, the
illumination on the second photograph is only one fourth as strong as
it was on a similar photograph held at a distance of one foot from the
candle.
The photograph or object placed at a distance of one foot from a light
is well illuminated; if it is placed at a distance of two feet, the
illumination is only one fourth as strong, and if the object is placed
three feet away, the illumination is only one ninth as strong. This
fact should make us have thought and care in the use of our eyes. We
think we are sixteen times as well off with our incandescent lights as
our ancestors were with simple candles, but we must reflect that our
ancestors kept the candle near them, "at their elbow," so to speak,
while we sit at some distance from the light and unconcernedly read
and sew.
As an object recedes from a light the illumination which it receives
diminishes rapidly, for the strength of the illumination is inversely
proportional to the square of distance of the object from the light.
Our ancestors with a candle at a distance of one foot from a book were
as well off as we are with an incandescent light four feet away.
101. Money Value of Light. Light is bought and sold almost as
readily as are the products of farm and dairy; many factories,
churches, and apartments pay a definite sum for electric light of a
standard strength, and naturally full value is desired. An instrument
for measuring the strength of a light is called a photometer, and
there are many different varieties, just as there are varieties of
scales which measure household articles. One light-measuring scale
depends upon the law that the intensity of illumination decreases with
the square of the distance of the object from the light. Suppose we
wish to measure the strength of the electric light bulbs in our homes,
in order to see whether we are getting the specified illumination. In
front of a screen place a black rod (Fig. 58) which is illuminated by
two different lights; namely, a standard candle and an incandescent
bulb whose strength is to be measured. Two shadows of the rod will
fall on the screen, one caused by the candle and the other caused by
the incandescent light. The shadow due to the latter source is not so
dark as that due to the candle. Now let the incandescent light be
moved away from the screen until the two shadows are of equal
darkness. If the incandescent light is four times as far away from the
screen as the candle, and the shadows are equal, we know, by Section
100, that its strength is sixteen candle power. If the incandescent
light is four times as far away from the screen as the candle is, its
power must be sixteen times as great, and we know the company is
furnishing the standard amount of light for a sixteen candle power
electric bulb. If, however, the bulb must be moved nearer to the rod
in order that the two shadows may be similar then the light given by
the bulb is less than sixteen candle power, and less than that due the
consumer.
[Illustration: FIG. 58.--The two shadows are equally dark.]
102. How Light Travels. We never expect to see around a corner, and
if we wish to see through pinholes in three separate pieces of
cardboard, we place the cardboards so that the three holes are in a
straight line. When sunlight enters a dark room through a small
opening, the dust particles dancing in the sun show a straight ray. If
a hole is made in a card, and the card is held in front of a light,
the card casts a shadow, in the center of which is a bright spot. The
light, the hole, and the bright spot are all in the same straight
line. These simple observations lead us to think that light travels in
a straight line.
[Illustration: FIG. 59.--The candle cannot be seen unless the three
pinholes are in a strait line.]
We can always tell the direction from which light comes, either by the
shadow cast or by the bright spot formed when an opening occurs in the
opaque object casting the shadow. If the shadow of a tree falls
towards the west, we know the sun must be in the cast; if a bright
spot is on the floor, we can easily locate the light whose rays stream
through an opening and form the bright spot. We know that light
travels in a straight line, and following the path of the beam which
comes to our eyes, we are sure to locate the light.
103. Good and Bad Mirrors. As we walk along the street, we
frequently see ourselves reflected in the shop windows, in polished
metal signboards, in the metal trimmings of wagons and automobiles;
but in mirrors we get the best image of ourselves. We resent the image
given by a piece of tin, because the reflection is distorted and does
not picture us as we really are; a rough surface does not give a fair
representation; if we want a true image of ourselves, we must use a
smooth surface like a mirror as a reflector. If the water in a pond
is absolutely still, we get a clear, true image of the trees, but if
there are ripples on the surface, the reflection is blurred and
distorted. A metal roof reflects so much light that the eyes are
dazzled by it, and a whitewashed fence injures the eyes because of the
glare which comes from the reflected light. Neither of these could be
called mirrors, however, because although they reflect light, they
reflect it so irregularly that not even a suggestion of an image can
be obtained.
Most of us are sufficiently familiar with mirrors to know that the
image is a duplicate of ourselves with regard to size, shape, color,
and expression, but that it appears to be back of the mirror, while we
are actually in front of the mirror. The image appears not only behind
the mirror, but it is also exactly as far back of the mirror as we are
in front of it; if we approach the mirror, the image also draws
nearer; if we withdraw, it likewise recedes.
104. The Path of Light. If a mirror or any other polished surface is
held in the path of a sunbeam, some of the light is reflected, and by
rotating the mirror the reflected sunbeam may be made to take any
path. School children amuse themselves by reflecting sunbeams from a
mirror into their companions' faces. If the companion moves his head
in order to avoid the reflected beam, his tormentor moves or inclines
the mirror and flashes the beam back to his victim's face.
If a mirror is held so that a ray of light strikes it in a
perpendicular direction, the light is reflected backward along the
path by which it came. If, however, the light makes an angle with the
mirror, its direction is changed, and it leaves the mirror along a new
path. By observation we learn that when a beam strikes the mirror and
makes an angle of 30° with the perpendicular, the beam is reflected in
such a way that its new path also makes an angle of 30° with the
perpendicular. If the sunbeam strikes the mirror at an angle of 32°
with the perpendicular, the path of the reflected ray also makes an
angle of 32° with the perpendicular. The ray (_AC_, Fig. 60) which
falls upon the mirror is called the incident ray, and the angle which
the incident ray (_AC_) makes with the perpendicular (_BC_) to the
mirror, at the point where the ray strikes the mirror, is called the
angle of incidence. The angle formed by the reflected ray (_CD_) and
this same perpendicular is called the angle of reflection. Observation
and experiment have taught us that light is always reflected in such a
way that the angle of reflection equals the angle of incidence. Light
is not the only illustration we have of the law of reflection. Every
child who bounces a ball makes use of this law, but he uses it
unconsciously. If an elastic ball is thrown perpendicularly against
the floor, it returns to the sender; if it is thrown against the floor
at an angle (Fig. 61), it rebounds in the opposite direction, but
always in such a way that the angle of reflection equals the angle of
incidence.
[Illustration: FIG. 60.--The ray _AC_ is reflected as _CD_.]
[Illustration: FIG. 61.--A bouncing ball illustrates the law of
reflection.]
105. Why the Image seems to be behind the Mirror. If a candle is
placed in front of a mirror, as in Figure 62, one of the rays of light
which leaves the candle will fall upon the mirror as _AB_ and will be
reflected as _BC_ (in such a way that the angle of reflection equals
the angle of incidence). If an observer stands at _C_, he will think
that the point _A_ of the candle is somewhere along the line _CB_
extended. Such a supposition would be justified from Section 102. But
the candle sends out light in all directions; one ray therefore will
strike the mirror as _AD_ and will be reflected as _DE_, and an
observer at _E_ will think that the point _A_ of the candle is
somewhere along the line _ED_. In order that both observers may be
correct, that is, in order that the light may seem to be in both these
directions, the image of the point _A_ must seem to be at the
intersection of the two lines. In a similar manner it can be shown
that every point of the image of the candle seems to be behind the
mirror.
[Illustration: FIG. 62.--The image is a duplicate of the object, but
appears to be behind the mirror.]
It can be shown by experiment that the distance of the image behind
the mirror is equal to the distance of the object in front of the
mirror.
106. Why Objects are Visible. If the beam of light falls upon a
sheet of paper, or upon a photograph, instead of upon a smooth
polished surface, no definite reflected ray will be seen, but a glare
will be produced by the scattering of the beam of light. The surface
of the paper or photograph is rough, and as a result, it scatters the
beam in every direction. It is hard for us to realize that a smooth
sheet of paper is by no means so smooth as it looks. It is rough
compared with a polished mirror. The law of reflection always holds,
however, no matter what the reflecting surface is,--the angle of
reflection always equals the angle of incidence. In a smooth body the
reflected beams are all parallel; in a rough body, the reflected beams
are inclined to each other in all sorts of ways, and no two beams
leave the paper in exactly the same direction.
[Illustration: FIG. 63.--The surface of the paper, although smooth in
appearance, is in reality rough, and scatters the light in every
direction.]
Hot coals, red-hot stoves, gas flames, and candles shine by their own
light, and are self-luminous. Objects like chairs, tables, carpets,
have no light within themselves and are visible only when they receive
light from a luminous source and reflect that light. We know that
these objects are not self-luminous, because they are not visible at
night unless a lamp or gas is burning. When light from any luminous
object falls upon books, desks, or dishes, it meets rough surfaces,
and hence undergoes diffuse reflection, and is scattered irregularly
in all directions. No matter where the eye is, some reflected rays
enter it, and the various objects are clearly seen.