TABLE OF CONTENTS The action of a lens on a wave front was briefly discussed in Sec. 1.4. Figures 1.8 and 1.9 showed how a lens can modify a wave front to form an image. A wave front is difficult to manipulate mathematically, and for most purposes the concept of a light ray (which is the path described by a point on a wave front) is much more convenient. In an isotropic medium, light rays are straight lines normal to the wave front, and the image of a point source is formed where the rays converge (or appear to converge) to a concentration or focus. In a "perfect" lens the rays converge to a point at the image.
For purposes of calculation, an extended object may be regarded as an array of point sources. The location and size of the image formed by a given optical system can be determined by locating the respective images of the sources making up the object. This can be accomplished by calculating the paths of a large number of rays from each object point through the optical system, applying Snell's law (Eq. 1.3) at each ray-surface intersection in turn. However, it is possible to locate optical images with considerably less effort by means of simple equations derived from the limiting case of the trigonometrically traced ray (as the angles involved approach zero). These expressions yield image positions and sizes which would be produced by a perfect optical system; they are paraxial or first-order.
