Name the defects of myopia as well as hypermetropia. What type of spectacles should be worn by a person having the defects of myopia as well as hypermetropia? How does it help?
Hint: The retina is like the focal point of our eye. Defects occur when images are formed in front of the retina (by rays converging earlier) or behind the retina (by rays converging beyond the focus) instead. This leads to a compromise in vision clarity. In each case think of what lens you would employ to curb the over-convergence or over-divergence of the light rays.
Complete step by step answer: Myopia and hypermetropia are in general, defects in vision caused by changes in the structure of the convex eye lens that is situated behind the pupil of the eye through which light enters. For clear vision, all the incoming light should converge at the retina that is located in the back of our eye. In other words, the retina acts like a focal point. Let us begin by understanding what each of the terms mean. Myopia, also known as near-sightedness, is the defect in which near objects appear clear whereas distant objects get unfocused and blurry. This happens when the eyeball gets elongated, following which the distance between the lens and the retina increases in such a way that the light from far off objects converges at a point before hitting the retina. Rectification of this involves introducing an additional lens that diverges the incoming rays just enough so that when our eye re-converges the light it does so by forming a virtual object that is much closer than the actual object and hits the retina. Thus, we use a diverging lens or a concave lens to rectify this defect. Hypermetropia, also known as far-sightedness, is the defect in which near objects seem blurry and unfocused whereas far-off objects appear focused and clear. This happens when the eyeball becomes smaller along its axis and the distance between the eye lens and the retina reduces. As a result, the rays coming from a nearby object meet at a point behind the retina and the object cannot be seen clearly. Rectification of this also involves introducing an additional lens that converges light just enough to compensate for the over-diverging nature of the eye-lens by forming a virtual object that is much further than the actual object and the image converges at the retina. Thus, we use a converging lens or a convex lens to rectify this defect.
Note: The extent of the above eye defects differs from person to person. Thus the focal length of the corrective lens varies for everyone, it is sometimes different from even one eye to the other. Also remember that decrease in the curvature of the eye lens corresponds to a longer focal length (hypermetropia, where image is focused beyond the retina) whereas increase in curvature leads to a shorter focal length (myopia, where image is focused before the retina).