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the water in the same path. If it enters the water on either side of this normal, it is refracted or
turned aside, and takes a new path. The greater the angle at which it enters the water, the greater the refraction. In the diagram the line in the middle represents the ray of light in the centre that meets the water at a normal, and passes straight through it and on into the air above. On either side the rays are represented as refracted, or bent out of their track, and obliged to take new paths. The greater the distance of the ray from the normal, the greater its refraction. Now, as all the rays at the same distance from the normal are refracted to the same degree, it follows that there must be a place where all these rays of refraction will meet.
Look at the cone of light over the bowl of water, and you can see the spot where all the rays of light are concentrated. Here they meet in what is called a focus. You can readily remember this word, because it means a hearth, or burning-place, and we saw our match take fire just at that point. The rays of sunlight contain heat as well as light, and, if we gather them altogether in a bundle, of course we shall concentrate both the heat and light. A bit of
A bit of paper held in the focus glows with intensely white light, and presently begins to smoke and burn in the concentrated heat.
This bowl of water is called a lens, and, by means of refraction, we may use it to concentrate light and heat. Beyond the focus you observe the light spreads out again till it meets the ceiling, where it appears as a broad disk of light. In the diagram each ray
is represented as meeting at the focus, and then all pass each other and go on in their previous directions; and you can readily see that a new cone of light will be formed, upside down, above the focus; and beyond it all the rays will spread out wider and wider the farther they go. Hold a piece of paper just above the focus, and you will see a small circle of light upon it. Raise it higher, and the circle grows larger and larger, and on the ceiling it is several feet wide.
By means of lenses of glass or water we can spread out a beam of light, gather a whole bundle of rays into a single focus—we can make distant objects appear near, make small things appear large, and large things appear small. Telescopes, microscopes, spectacles, and all kinds of optical instruments, are founded on this simple law of refraction, as shown by this bowl of water.
EXPERIMENTS IN PROJECTION.
At the optician's you can purchase a small glass plano-convex lens, 3 inches (76 millimetres) in diameter, and of a focal length of about 8 inches, for perhaps less than fifty cents. Such a lens is flat on one side and convex on the other, and from this it takes its name. Take this lens into a room, and close the curtains at all the windows save one. Soften a piece of wax, and stick the lens into it, so that it will serve as a handle, and then hold the lens a few inches from the wall, or, if the wall is dark-colored, before a sheet of paper pinned upon the wall, and just opposite the
window. On the wall will then appear a picture of the window, and the trees, houses, and other objects that may be seen through it. Move the lens backward or forward, and you will find a place where the image on the wall becomes distinct, and gives a miniature view of the window in its natural colors, and upside down.
Here the light from the window falls upon the lens, and is refracted to a focus. This focus consists of points, each of which is formed by the convergence of rays which come from a similar point in the window. This we call a projection, because the light is projected or thrown upon the wall by the lens. To understand this we must notice that every part of the window sends light into the room in every direction. Every part sends light into the entire lens, and each beam is refracted and takes a new direction be
yond it, so that, ultimately, all the rays meet at the focus. In examining this projection, you will notice that the glass is quite near the wall when the focus is clear and sharp. If we measure the distance from the lens to the projection, we shall get a certain measurement. This measure we call the focal distance of the lens.
Fig. 22 represents two rays from the top of the window and two from the bottom, and shows the