CBSE Class 10 Science 2017 · Paper 31/1/3 · Question 17

Power of accommodation: the eye lens is flexible, and the ciliary muscles can change its curvature and so its focal length. The ability of the eye lens to adjust its focal length so that objects at different distances are focused sharply on the retina is called the power of accommodation. For a distant object the ciliary muscles relax, the lens becomes thin and its focal length increases; for a nearby object they contract, the lens becomes thicker and its focal length decreases. A normal eye can focus objects from about 25 cm (the near point) to infinity (the far point). Why the image distance does not change: for an object to be seen clearly its image must be formed on the retina, and the distance between the eye lens and the retina is fixed by the size of the eyeball. So the image distance cannot change; when the object distance changes, the eye keeps the image on the retina by changing the focal length of its lens (accommodation), not by changing the image distance.