CHAPTER 8

Physical Optics

This chapter discusses the characteristics of light and some of the ways these characteristics can be put to clinical use.

The Corpuscular Theory of Light

The Greek philosophers Leucippus and Democritus (c 450 BC) proposed that light is a flow of miniscule particles or “corpuscles.” Such particles, it was thought, travel instantaneously between points separated by any distance, which is to say at infinite speed.

According to the law of rectilinear propagation, light moves in straight lines and changes direction only if reflected or refracted (Fig 8-1). The law of reflection was established by observing light rebounding symmetrically from smooth surfaces (Fig 8-2). Refraction was also readily observed but, at the time, not as clearly understood (Fig 8-3). The angles of incidence and transmission (the latter also known as the angle of refraction) were thought to be proportional, which is not correct but is a good approximation for small angles.

Figure 8-1 Sunlight passing through openings between partially overlapping leaves suggests that light moves in straight lines. However, such “rays” would be invisible without light scattering caused by mist in the air (eg, Tyndall effect). Likewise, the beam of a slit lamp is not normally visible in the anterior chamber but becomes so when light is scattered by the presence of abnormal proteins or cells. (Illustration b y Edmond H. Thall, MD.)

Figure 8-2 The law of reflection states that an incident ray rebounds symmetrically after striking a smooth surface. The angles of incidence (θi) and reflection (θr) are defined with respect to an imaginary line, the surface normal. The law states that (1) the incident ray, reflected ray, and surface normal are coplanar, and (2) θi equals θr. (Illustration b y Edmond H. Thall,

MD.)

Figure 8-3 Light moving from one material to another abruptly changes speed (refraction) and usually direction. The angles of incidence (θi) and transmission (θt) are defined with respect to the surface normal. The incident ray, refracted ray, and surface normal are coplanar. θi and θt are related by the equation n1 sin θi = n2 sin θt. The approximation n1 θi = n2 θt is sufficiently accurate for small angles. (Illustration b y Edmond H. Thall, MD.)

The corpuscular hypothesis lacked supporting experimental evidence. Nevertheless, it was accepted essentially unchallenged for 2000 years.

Diffraction

According to the corpuscular theory, collimated light should remain so after traversing an aperture. Actually, the beam diverges, violating the law of rectilinear propagation (Fig 8-4). All apertures produce diffraction to some extent. The smaller the aperture is relative to the wavelength, the more pronounced is the consequent diffraction (Fig 8-5).

Figure 8-4 Behavior of light crossing an aperture. A, According to the corpuscular theory of light, a collimated beam should remain so after crossing an aperture that blocks the sides of the beam. B, Instead, the beam diverges. This phenomenon, called diffraction, violates the law of rectilinear propagation and suggests that light is a wave, not a series of particles (corpuscular).