CBSE Class 10 Science 2017 · Paper 31/1 · Question 24

(a) Possible reasons: (1) excessive curvature of the eye lens (too short a focal length); (2) elongation of the eyeball. (i) The far point of the student's eye is $5$ m. Rays from an object at $5$ m are focused on the retina, but rays from an object beyond $5$ m are brought to a focus in front of the retina and reach it as a blurred patch, so such objects are not seen distinctly. (ii) A concave (diverging) lens of suitable power is used. It spreads out the rays from a distant object so that they appear to come from the far point, $5$ m away; the eye can focus such rays on the retina, so the image is formed on the retina and proper vision is restored. (b) For the concave lens $f=-5$ m, so $P=\frac{1}{f}=\frac{1}{-5}=-0.2$ D.