CBSE Class 10 Science 2017 · Paper 31/2/1 · Question 24

(a)(i) Cornea: the thin transparent membrane at the front of the eye through which light enters; most of the refraction of the light entering the eye occurs at its outer surface. (a)(ii) Iris: a dark muscular diaphragm that controls the size of the pupil, and so regulates the amount of light entering the eye. (a)(iii) Crystalline (eye) lens: a transparent, flexible convex lens that provides the finer adjustment of focal length needed to focus objects at different distances on the retina, where a real, inverted image is formed. (a)(iv) Ciliary muscles: they hold the eye lens and change its curvature, and hence its focal length (relaxed: lens thinner, for distant objects; contracted: lens thicker, for near objects). This is accommodation. (a)(v) Retina: the light-sensitive screen on which the image is formed; its light-sensitive cells produce electrical signals, which the optic nerve carries to the brain. (b) The person suffers from hypermetropia (far-sightedness, or long-sightedness): the near point has moved from about 25 cm out to 1 m. Cause: the focal length of the eye lens is too long, or the eyeball has become too small, so light from a nearby object (at 25 cm) is brought to a focus behind the retina (first ray diagram). Correction: a convex (converging) lens of suitable power, which makes light from an object at 25 cm appear to come from the near point at 1 m, so that the eye can focus it on the retina (second ray diagram). For a lens close to the eye this needs $f\approx +33$ cm, a power of about $+3$ D.