This chapter introduces certain physical principles that are important to an understanding of how images are formed by optical systems. We then treat the structures and mechanisms in the human eye that produce the retinal image and determine its quality.

It is useful to begin by briefly discussing light as an oscillating electromagnetic field, because many properties of optical systems such as that of the eye depend on the nature of these fields. An electromagnetic field arises from the motion of a charge and propagates away in all directions perpendicular to the charge's axis of movement. Figure 3.1 A illustrates this schematically for a charge oscillating sinusoidally in the plane of the page. The sinusoidal wave depicted represents only the electric field produced by the moving charge; at right angles to the electric field, normal to the plane of the page, is an oscillating magnetic field. Seen from above (Figure 3.IB), the electromagnetic field, represented here by the positive peaks of its electric component, propagates away from the charge as a succession of curved wave fronts that can be treated as planar at some distance from the source (right side of Figure 3.IB). Depending on the situation, it is convenient to think of light either as an advancing wave front or as a ray, the latter usually being represented by an arrow normal to the advancing wave front.

Scatter, Interference, and Transparency

When a propagating wave front of light encounters matter, charges in the matter experience the alternating electromagnetic field, and some are set in motion.