LIGHT.
As I told you about heat, that we do not know what it is, so, also, we
do not know what light is. But we know many things about light, just as
we do about heat.
[Sidenote: The chief use of light.]
The chief use of light is to enable us and different animals to see. I
have told you something about seeing in Part Second. It is the light
entering the eye that makes us see. When we see the sun, or the flame
of a candle, or a flash of lightning, the light which is made by these
different things goes into the eye, and so we see them.
These things that I have mentioned make light, and some of this light
comes directly to our eyes. But we see things that do not make any
light. No light is made by the houses, and trees, and persons, and many
other things that we see about us. How is it that we see them? It is in
this way: The light that shines on them bounds off from them and goes
into our eyes. Thus, if you see a tree, the light strikes upon it, and
then bounds from it into your eyes, and makes a picture or image there
of the tree. When the light bounds off in this way, it is said to be
_reflected_.
[Sidenote: Reflection of light.]
[Sidenote: Images of things in the eye.]
There is a great deal of this reflection of light. It is often
reflected more than once, sometimes many times. Thus, if you see a tree
in a looking-glass, the light is reflected twice. First, it bounds off
or is reflected from the tree, and then it is reflected from the glass
to your eyes. So if you look at your own face, the light first strikes
your face, and is reflected from it to the glass; and then it is
reflected from the glass to your eyes, and pictures the image of your
face there.
Now observe that the light that is reflected from your face makes an
image or picture of it in the glass. It is precisely such an image that
the light entering your eye makes in the back part of it, on a thin
sheet or membrane that is there, except that it is a much smaller image.
[Sidenote: Smooth and rough things.]
Every thing reflects light, but some things reflect it more than
others. Rough things do not reflect as much as smooth things. How
perfectly the smooth water of a pond reflects the houses and trees at
its side when there is no wind! You know that all polished surfaces
shine. This is because they reflect a great deal of light.
[Sidenote: Moon and stars.]
It is a reflected light that comes to us from the moon and from some of
the stars. The light goes to them from the sun, and then is reflected
from them. They are said, therefore, to shine by a borrowed light. The
reason that we can not see the stars in the daytime is, that the light
from the sun is so much brighter than their light. The moon shines so
much more brightly than the stars, that we can see it in the daytime
when it is above the horizon, though the greater brightness of the sun
makes it quite faint.
I have told you that light is sometimes reflected more than twice, even
many times. When you look at a person in a room into which the sun is
not directly shining, where does the light by which you see him come
from? It is not the light that comes straight from the sun, for this is
not shining upon him. It is the light reflected from things around him.
This reflected light strikes upon him, and is thus again reflected from
him, and some of it enters your eyes, enabling you to see him.
[Sidenote: Light reflected back and forth.]
Light is thus reflected back and forth from one thing to another; and a
great deal of light is reflected from every thing all the time, and in
all directions. Suppose a great assembly are all looking at one person.
The light is reflected from him, and goes into a thousand eyes at once
in all parts of the house, making a picture of him in all of them.
What a wonderful painter light is! How many pictures it is making all
the time in the eyes of men and animals, and on mirrors and all smooth
things every where!
[Sidenote: Light makes plants and animals grow.]
Another use of light is to make plants and animals grow. I have told
you in Part I. how plants turn toward the light, as if they loved it.
It really has a great deal to do with their growth.
This is very plain whenever we see a plant that has grown in the dark.
It looks pale and sickly. A good deal of light is needed as really as a
free circulation of air to make plants healthy and strong; and the same
is true of animals. People that live in dark, under-ground rooms in
cities are injured by the want of light as well as by the want of good
air.
Most of the light in the world comes from the sun. It comes from there
with the heat, as I have before told you. They travel in company. It
is a very long journey. It is many millions of miles. The light is a
little more than eight minutes coming from the sun to the earth.
[Sidenote: Light travels faster than sound.]
Light travels very fast. It travels faster than sound does. You see a
man cutting wood a considerable distance off, and you hear the sound of
each blow of his axe a little after you see it. The reason is that the
light comes from him to your eye quicker than the sound comes to your
ear. You see a cannon fired at a distance; you first see the flash,
and then afterward hear the report. The thunder comes generally some
time after the flash that causes it; that is, the light of the flash
gets to your eye some time before the sound of it reaches your ear. By
observing, it has been found out just how fast sound and light travel;
and so, by looking at a watch in a thunder-storm, we can tell how far
off the lightning is.
Light, besides traveling faster than sound, can travel a great deal
farther. Lightning may be so far off that you can not hear the thunder.
The light reaches your eye, but the sound dies away before it reaches
your ear.
[Sidenote: Light of burning substances.]
Most of our light, I have said, comes from the sun; but much light
comes from burning substances--burning wood, coal, oil, tallow, gas, &c.
[Sidenote: Fire-flies.]
[Sidenote: Shining flowers.]
[Sidenote: Light-wood.]
Light is made by some animals. The glow-worm gives out a soft and
beautiful light. The fire-fly sparkles as it flies about in the
evening. In Cuba and in South America ladies wear in their hair as
ornaments, in evening parties, some small insects that give a very
brilliant light. Sometimes the sea sparkles beautifully with light,
which is made by multitudes of very little animals in it. We see
this light often in the wake of a vessel, or behind the wheel of the
steamer, or in the water that falls from the lifted oar. It is when
the water is disturbed in some way that these animals make their light.
There are some flowers in very warm countries that shine in the night.
You have seen what is called light-wood. This is decayed wood, and it
is something in the decay that makes the light. Light is also sometimes
given out by animal substances that are decaying. It is most often seen
in putrid fish.
[Sidenote: Phosphorus.]
It is supposed that in all these cases the light is made by phosphorus,
the same substance that lights so easily in the Lucifer match. This
curious substance is commonly kept in water. If a stick of it be taken
out of the water in the evening, it appears lighted like a glow-worm;
and if you rub it upon any thing, the streaks of it will give a
brilliant white light. Sometimes, on rubbing a match, if it does not
take fire, you see for a little time lighted streaks where you rubbed
it. This is caused by the phosphorus rubbed off from the match. When
the match burns, you do not see these lighted streaks, for the same
reason that you do not see the stars when the sun shines.
_Questions._--What is the chief use of light? How do we see? How
do we see things that do not make light? How do we see things in a
mirror? How is the image in the mirror like that in the eye? What
difference is there in things in reflecting light? What is said about
the light of the moon and the stars? Why can not we see the stars in
the daytime? Why can we see the moon in the daytime? What is mentioned
which shows that light is often reflected many times before it comes
into the eye? Tell what is said about an assembly all looking at a
speaker. What effect has light upon plants and animals? What is said
about living in dark rooms? How long is light in coming from the sun?
Give some examples which show that it travels faster than sound. Can
sound go as far as light? From what besides the sun does light come?
Tell about the fire-flies--the sparkling that we often see in the
sea--light-wood. What is said about phosphorus?