CBSE Class 10 Science 2019 · Paper 31/1/1/C · Question 20(a)

(a) The ciliary muscles hold the eye lens and change its curvature, and hence its focal length. When they relax, the lens becomes thin and its focal length increases, so distant objects are seen clearly; when they contract, the lens becomes thicker and its focal length decreases, so nearby objects are seen clearly. This ability of the eye lens to adjust its focal length is called accommodation. (b) (i) Cataract: corrected by cataract surgery, in which the cloudy lens is removed and replaced by an artificial lens. (ii) Myopia (near-sightedness): corrected by a concave (diverging) lens of suitable power. (iii) Hypermetropia (far-sightedness): corrected by a convex (converging) lens of suitable power. (iv) Presbyopia: corrected by a convex lens for near vision, or by bifocal lenses (upper part concave, lower part convex) when distant vision is also affected.