The soft variety cannot corrnect astigmatism and the vision may not be satisfactorily corrected Reduced eye (also called 'schematic eye) It has

nodal point of eye. Normally, the nodal point is 17 mm (= 17/1000 m) in front of the retina. Retina being the structure where the object is focussed (as image), he focus lies on Ihe retina and the focal length is obviously 17 mm. and the power of the optical system of normal eye (simply, power of the eye), is, or 59D (see also. the reduced eye. 1 17/1000 later this chap) Of this 55 D, comea alone accounts for about 43 D and the lens (when removed from the body) accounts for about 16D.But in vivo the lens accounts for aboul 10 D only. Fig. 10B2. 1. 4. To show that the power of lens increases as the curvature of I ens increases It should be understood that refractive power of the crystalline lens can be altered to good exlenl by the process of accomodatibn Further, as fig. 10B2. 1. 4 shows, the power of the lens increases as the curvature of the lens increases With the increase of power of the lens, the focus, where the image is formed, is brought nearer the lens. Concave lens So long all discussions centered round Ihe convex lens, because the crystalline lens of the eye is a convex lens. However, some considerations of concave lens is also required at this stage. A concave lens, is a diverging lens (in contrasl lo convex lans, convex lens being converging lens as shown in fig. 10B2.1. 4). Focal point, or the focus, therefore, in a concave lens is an artificial point as shown in fig.10B2.1. 5. Fig-10B2-1. 5 To show that a concave lens is a diverging lens By convention, power of concave and convex lenses are symbolised by - and + signs respectively Thus. 5D and + 50 denote, a concave and a convex lens respectively, having 5D power. Contact lenses are becoming very popular. Such lenses are fitted on to the curvature of cornea. Two varieties are available, viz. (i) hard, and (ii) soft. In the hard variety, vision is stable and the lens can take care of astigmatism, but comeal scratches can develop. The soft variety cannot corrnect astigmatism and the vision may not be satisfactorily corrected Reduced eye (also called 'schematic eye) It has