CBSE Class 10 Science 2018 · Paper 31/1 · Question 20(a)

(a) The boy is suffering from myopia (near-sightedness): distant objects, such as the words on a board $3$ m away, cannot be seen distinctly, though nearby objects can. The image of a distant object is formed in front of the retina. Possible causes: (i) excessive curvature of the eye lens (the eye lens becomes too converging); (ii) elongation of the eyeball. Correction: myopia is corrected by using a concave (diverging) lens of suitable power, whose focal length equals the distance of the far point. The concave lens makes the rays from a distant object diverge as if they came from the far point of the eye, so the eye lens can form the image on the retina (ray diagram: without the lens the rays meet in front of the retina; with the concave lens they meet on the retina). (b) Stars twinkle because of atmospheric refraction of starlight. Starlight passes through layers of air of varying density (refractive index) before reaching us and is bent, so the star appears at a position slightly different from (higher than) its actual position. As the layers of air keep changing, the apparent position of the star keeps changing and the amount of starlight entering the eye flickers, so the star appears to twinkle.