CBSE Class 10 Science 2020 · Paper 31/C/1 · Question 28

(a) Myopia (near-sightedness), in which the image of a distant object forms in front of the retina; its causes are excessive curvature of the eye lens (its focal length becomes too short) and elongation of the eyeball. Hypermetropia (far-sightedness), in which the image of a nearby object forms behind the retina; its causes are too long a focal length of the eye lens and an eyeball that has become too small. (Presbyopia, caused by the weakening of the ciliary muscles and the diminishing flexibility of the eye lens, may be named instead of either.) (b) $P=\frac{1}{f}=\frac{1}{-2.0\text{m}}=-0.5$ D. The focal length and the power are negative, so the lenses are concave (diverging) lenses of power $-0.5$ D; they correct myopia.