Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 110

An object that bends (or refracts) light is called a lens. When a single ray of light strikes a lens, the ray undergoes simple refraction, as depicted in Figure 2.9. Most objects or images, however, generate a pencil of light rays rather than a single ray. When a pencil of rays strikes a lens, they spread apart (i.e., diverge) or come together (i.e., converge). Convergence, or positive vergence, occurs when light strikes a convex lens (Figure 2.10a–c). Such a lens has a positive power, indicating that it forms a real image, which means that incoming rays from the object are converged and focused on the other side of the lens (see Figure 2.10a and b). On the other hand, divergence, or negative vergence, occurs when light strikes a concave lens (see Figure 2.10c). The negative power of the concave lens indicates that it forms a virtual or aerial image, which means that the diverging rays are traced, using imaginary extensions, backward to a “focused” virtual image “located” on the same side of the lens as the object (dashed, “imaginary” lines). The vergence power (i.e., amount of bending) of a lens is measured in units called diopters. One diopter (D) is the vergence power of a lens with a focal length (f) of 1 m when in air.

Figure 2.9 Refraction of light as it passes from one medium to another is governed by Snell's law, summarized in the formula below the diagram. The angle of refraction ( θ _r) is a function of the angle of incidence ( θ _i) and the refractive indices of the two media. In this representation, n _i < n _r; therefore, θ _i > θ _r.