18 - Refraction and Accommodation

Abstract

Publisher Summary This chapter provides an overview of refraction and accommodation. Refractive power of optical lenses is measured in dioptres (D) and is the reciprocal of the focal length (f) in meters of a lens. A lens with a refractive power of two dioptres has a focal length of half metre. Convex lenses cause parallel incident light rays to converge to a single focal point beyond the lens. Concave lenses cause incident rays to diverge; the focus is, therefore, in front of the lens. Light consists of electromagnetic vibrations, and the visible spectrum is limited to a wavelength range of approximately 400 to 800 nm in man. White light is split into its constituent spectral colors by prisms. The refractive power at the edge of both the crystalline lens and the cornea is not the same as at the center and, therefore, gives rise to spherical and chromatic aberrations. Blue light is refracted more than red, which has the longer wavelength. In hypermetropia, parallel rays of incident light come to a focus behind the retina, producing a blurred retinal image. That may be because the eye is too short in proportion to its refractive power, the extreme example being microphthalmos. In myopia, parallel rays of incident light come to a focus in front of the retina. Only rays from near objects, which diverge, come to a focus on the retina. Anisometropia is a term used to describe the condition when the two eyes have unequal refractive powers. Objective refraction is performed by means of an instrument somewhat similar to an ophthalmoscope. Accommodation is the ability of the eye to augment its refractive power by increasing the curvature and, hence, the strength of the crystalline lens.