FIG. 70.—Rays of light are diverged and do not come to any real focus.
If two prisms are arranged as in Figure 70, the ray A will be refracted upward toward the thick end, and the ray B will be refracted downward toward the thick end; the two rays, on emerging, will therefore be widely separated and will not intersect.
Lenses are very similar to prisms; indeed, two prisms placed as in Figure 69, and rounded off, would make a very good convex lens. A lens is any transparent material, but usually glass, with one or both sides curved. The various types of lenses are shown in Figure 71.
FIG. 71.—The different types of lenses.
The first three types focus parallel rays at some common point F, as in Figure 69. Such lenses are called convex or converging lenses. The last three types, called concave lenses, scatter parallel rays so that they do not come to a focus, but diverge widely after passage through the lens.
113. The Shape and Material of a Lens. The main or principal focus of a lens, that is, the point at which rays parallel to the base line AB meet (Fig. 71), depends upon the shape of the lens. For example, a thick lens, such as A (Fig. 72), focuses the rays very near to the lens; B, which is not so thick, focuses the rays at a greater distance from the lens; and C, which is a very thin lens, focuses the rays at a considerable distance from the lens. The distance of the principal focus from the lens is called the focal length of the lens, and from the diagrams we see that the more convex the lens, the shorter the focal length.
FIG. 72.—The more curved the lens, the shorter the focal length, and the nearer the focus is to the lens.