If two prisms are arranged as in Figure 69, and two parallel rays of light fall upon the prisms, the beam A
will be bent downward toward the thickened portion of the prism, and the beam B
will be bent upward toward the thick portion of the prism, and after passing through the prism the two rays will intersect at some point F
, called a 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, 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
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.
FIG. - Rays of light are converged and focused at F
FIG. - Rays of light are diverged and do not come to any real focus.