Light is famous for being the fastest thing in the universe. Everybody knows that nothing can go faster than the speed of light. However, the speed of light does not always stay the same. It travels at its fastest in the vacuum of outer space. It travels almost as fast through air but a lot slower through glass. When light travels from one substance into another, it does strange things because of this change in speed. Lenses are devices built to take advantage of these properties of light. Lenses come in two basic types: convex and concave. Whereas convex lenses magnify the images of things seen through them, concave lenses shrink the images.
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To understand why concave lenses reduce the sizes of images seen through them, you have to understand about normal lines. A “normal” line is a line that stands straight up out of a surface at ninety degrees to that surface. A flagpole, for instance, is on a normal line to the ground.
The Concave Lens
A concave lens is a thin piece of glass (or sometimes plastic) with two hollow curved surfaces. It is like a flat piece of glass with a shallow bowl in each side. Because the surface of the lens is curved like that, the normal lines at each point on the lens are slanted more to the side, pointing more toward the centre the closer they are to the rim of the lens.
A lens works by means of the refraction of light. Light slows down when it enters a denser substance like glass because there's more for it to run into in glass than in air. This makes a light beam bend more toward the normal line of the surface of the glass.
This is because as one side of a beam of light hits the glass, it is slowed while the other side keeps trying to move along at the same speed. It is rather like travelling in a car when its wheels go off the road and onto a gravel shoulder. The wheels in the gravel go slower than the ones on the tar do, so the car turns off the road entirely. It has moved toward the “normal line” of the road.
How It Works
When light moves from a denser substance like glass to a lighter substance like air, exactly the reverse of this happens. It bends away from the normal line. This is the key to understanding how concave lenses work. Because all the normal lines on the surface of a concave lens point inward, the light rays spread out when they enter the lens, travelling more away from each other. When the light leaves on the opposite side, it bends away from the normal lines. However, because the other side is also bowl-shaped but facing in the other direction, the normal lines all slant the other way. So the rays spread out yet again.
This makes the light rays that form the image slope at wider angles relative to each other, which gives the eye the impression that they have come from a smaller image seen up close. Concave lenses are used to see more in a single field of vision than would normally fit into it.
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