. As age further advances the lens lose water and begins to become hard so that ability to bulge as stated in the preceding para, deteriorates.

This is the fundamental cause of'presbyopia'(see, for further details, later). In addition, lens, at old age, becomes opaque- a change which is probably hastened by strong sun rays and exposure to ultraviolet rays. When the opacity is sufficient, the lens is said to have developed 'cataract' The vireous humor is an amorphous, highly viscid, gel like substance, occupying the big space behind the lens. The retma has been described later. OPTICS OF THE EYE When a ray of lighl passes from one medium to another having a different refractive index (u). it suffers refractrorts or bending. Greater the difference of The refractive indices, greater is Ihe refraction. For pradical purposes, the refractive index of Ihe air may be presumed lo be 1, and those of tha cornea, aqueous humor and vitreous humor around 1. 33 and that of The crystalline lens aboul 1 4 if follows that for practical purposes, it can be presumed, that the light that enters the eye suffers refraction twice, when (i) it enters the cornea and (ii) when it enters the lens Power of a lens Greater the power of a lens, greater is its ability to bend (refract) the rays entering it. The power of a lens is expressed in terms of diopter (0). By definition.isdiopter is, 1 Focal length of the lens in meter Focal length All parallel rays (excepting the one that strikes the optical center of the lens) that strike a biconvex Lens, as shown in fig. 1OB2. 1.4 are refracted and converge to a point. This point where the rays converge is called the locus (or, Ihe principal focus). The distance between the optical center of the lens and the focus is called the focal length [N B • By definition, optical center is that point of the lens, through which passing light suffers no refraction (see the accompanying diagram] . In human eye, there are. as staled already, several slruclures (e g. cornea. the crystalline lens etc), which act as lens, i.e.. human eye has a compound lens system. The center of all These lenses (i.e the center of the oplica! system of an eye) is called