CBSE Class 10 Science 2017 · Paper 31/1/2 · Question 17

(a) Distant object: the ciliary muscles relax, the eye lens becomes thin, its curvature decreases and its focal length increases (it is longest). (b) Nearby objects: the ciliary muscles contract, the eye lens becomes thicker, its curvature increases and its focal length decreases. Why not nearer than 25 cm: the focal length of the eye lens cannot be decreased below a certain minimum, because the ciliary muscles cannot make the lens more curved beyond a limit. For a normal eye this shortest focal length focuses an object at about 25 cm (the near point, or least distance of distinct vision) on the retina. An object placed nearer would need a still shorter focal length, which the lens cannot provide, so its image would form behind the retina and look blurred, and the eye strains in trying to focus it.