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.
Reflection of light.
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.
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.
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.