## Physical Principles of Optics

Before it is possible to understand the optical system of the eye, the student must first be thoroughly familiar with the basic principles of optics, including the physics of light refraction, focusing, depth of focus, and so forth. A brief review of these physical principles is presented; then the optics of the eye is discussed.

### Refraction of Light

Refractive Index of a Transparent Substance. Light rays travel through air at a velocity of about 300,000 km/sec, but they travel much slower through transparent solids and liquids. The refractive index of a transparent substance is the ratio of the velocity of light in air to the velocity in the substance. The refractive index of air itself is 1.00. Thus, if light travels through a particular type of glass at a velocity of 200,000 km/sec, the refractive index of this glass is 300,000 divided by 200,000, or 1.50.

Refraction of Light Rays at an Interface Between Two Media with Different Refractive Indices.

When light rays traveling forward in a beam (as shown in Figure 49-1A) strike an interface that is perpendicular to the beam, the rays enter the second medium without deviating from their course. The only effect that occurs is decreased velocity of transmission and shorter wavelength, as shown in the figure by the shorter distances between wave fronts.

If the light rays pass through an angulated interface as shown in Figure 49-15, the rays bend if the refractive indices of the two media are different from each other. In this particular figure, the light rays are leaving air, which has a refractive index of 1.00, and are entering a block of glass having a refractive index of 1.50. When the beam first strikes the angulated interface, the lower edge of the beam enters the glass ahead of the upper edge. The wave front in the upper portion of the beam continues to travel at a velocity of 300,000 km/sec, while that which entered the glass travels at a velocity of 200,000 km/sec. This causes the upper portion of the wave front to move ahead of the lower portion, so that the wave front is no longer vertical but angulated to the right. Because the direction in which light travels is always perpendicular to the plane of the wave front, the direction of travel of the light beam bends downward.

This bending of light rays at an angulated interface is known as refraction. Note particularly that the degree of refraction increases as a function of (1) the ratio of the two refractive indices of the two transparent media and (2) the degree of angu-lation between the interface and the entering wave front.

### Application of Refractive Principles to Lenses

Convex Lens Focuses Light Rays. Figure 49-2 shows parallel light rays entering a convex lens. The light rays passing through the center of the lens strike the lens exactly perpendicular to the lens surface and, therefore, pass through the lens without being refracted. Toward either edge of the lens, however, the light rays strike a progressively more angulated interface. The outer rays bend more and more toward the center, which is called convergence of the rays. Half the bending occurs when the rays enter the lens, and half as they exit from the opposite side. (At this time, the student should pause and analyze why the rays bend toward the center on leaving the lens.) Finally, if the lens has exactly the proper curvature, parallel light  Figure 49-1

Light rays entering a glass surface perpendicular to the light rays (A) and a glass surface angulated to the light rays (B). This figure demonstrates that the distance between waves after they enter the glass is shortened to about two thirds that in air. It also shows that light rays striking an angulated glass surface are bent. Figure 49-3

Bending of light rays at each surface of a concave spherical lens, showing that parallel light rays are diverged. 