A camera obscura or a pinhole camera forms images on a screen, but the images are faint and not perfectly focused. The ability to create a bright, well-focused image is vastly improved by using a lens. A lens is a transparent object that uses refraction at curved surfaces to form an image from diverging light rays. We will defer a mathematical analysis of lenses until the next section. First, we want to establish a pictorial method of understanding image formation. This method is called ray tracing.

FIGURE 34.27 shows parallel light rays entering two different lenses. The left lens, called a converging lens, causes the rays to refract toward the optical axis. The common point through which initially parallel rays pass is called the focal point of the lens. The distance of the focal point from the lens is called the focal length $f$ of the lens. The right lens, called a diverging lens, refracts parallel rays away from the optical axis. This lens also has a focal point, but it is not as obvious.

FIGURE 34.27 Parallel light rays pass through a converging lens and a diverging lens.

FIGURE 34.28 clarifies the situation. In the case of a diverging lens, a backward projection of the diverging rays shows that they appear to have started from the same point. This is the focal point of a diverging lens, and its distance from the lens is the focal length of the lens. In the next section we’ll relate the focal length to the curvature and index of refraction of the lens, but now we’ll use the practical definition that the focal length is the distance from the lens at which rays parallel to the optical axis converge or from which they diverge.

FIGURE 34.28 The focal lengths of converging and diverging lenses.

FIGURE 34.28 Full Alternative Text