CBSE Class 10 Science 2019 · Paper 31/4/1 · Question 17

(a) The person is suffering from hypermetropia (far-sightedness): the near point has moved away to 50 cm. Two possible causes: the focal length of the eye lens is too long (the eye lens has become less converging), or the eyeball has become too small (too short). (b) In the ray diagram of the defect, rays from an object at $N^{\prime }$, 50 cm away, meet on the retina, but rays from an object at $N$, 25 cm away, would meet behind the retina, so its image is blurred. (c) A convex (converging) lens. With $u=-25$ cm and $v=-50$ cm: $\frac{1}{f}=\frac{1}{v}-\frac{1}{u}=-\frac{1}{50}+\frac{1}{25}=\frac{1}{50}$, so $f=+50$ cm $=+0.5$ m and $P=\frac{1}{f}=+2$ D. (d) In the labelled correction diagram, a convex lens of power $+2$ D placed in front of the eye bends the rays from an object at $N$ (25 cm) so that they appear to come from $N^{\prime }$ (50 cm); the eye lens then focuses them on the retina.