CBSE Class 10 Science 2017 · Paper 531 · Question 17

(a) To see a distant object, the ciliary muscles relax and the eye lens becomes thin: its curvature decreases and its focal length increases (it is then at its longest), so that parallel rays from the distant object are focused on the retina. (b) To see a nearby object clearly, the ciliary muscles contract and the eye lens becomes thicker: its curvature increases and its focal length decreases, so that the image of the near object still forms on the retina. Reason: the focal length of the eye lens cannot be decreased below a certain minimum limit, because the lens can be made only so curved. For a normal eye this limit corresponds to an object about 25 cm away (the near point, or least distance of distinct vision). An object closer than 25 cm would need a still shorter focal length; its image would fall behind the retina, so it looks blurred and the eye is strained.