Light that is emitted from a surface, or reflected from it, leaves the surface in the form of spherical wavefront's. Every point on the surface can be thought of as a source of these wavefront's. Rather than drawing the wavefront's, we customarily illustrate the propagation of light with rays. These are just lines with arrowheads that point in the direction in which the light is traveling. Lenses are objects made from a transparent material such as glass that, because of the shape of their surfaces, alter the shape of the wavefront's, and, therefore, the direction of the rays of light passing through them. There are two basic types of lenses.
These are: (1) Converging lenses that are thicker in the middle than at the edges and cause rays to converge more on the exit than on the entrance side of the lens, and diverging lenses that are thicker at the edges and cause rays to diverge more on the exit side. The surfaces of lenses are usually small segments of large spheres. A line drawn through the centers of these spheres will also go through the center of the lens and is called the axis of the lens. All rays that enter a converging lens traveling parallel to its axis will be converged on the opposite side at a point called the principle focus or focal point as illustrated in figure 9.
1 a. For a diverging lens, parallel rays will be diverged outward so they appear to have originated at the focal point. The focal length of a lens is the distance from the lens to the focal point. Since light may pass through a lens in either direction, there is a focal point, symmetrically located, on each side. The focal length of a lens is determined during manufacture and depends on the radius of curvature of each of its sides and the index of refraction of the glass it is made from.