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What change is made in the eye to enable it to focus on objects situated at different distances? Illustrate your answer with the help of diagrams.
The change that is made in the eye to enable it to focus on objects situated at different distances is that the ciliary muscles change the focal length (or converging power) of the eye lens to focus nearby as well as distant objects on the retina.
(a) When the eye is focussing on a distant object, it is said to be unaccommodated, as it is the relaxed state of the eye.
To focus on distant objects, the ciliary muscles of the eye get fully relaxed due to which they tightly pull the suspensory ligaments, which in turn pull the eye lens and it becomes thin. The thin eye-lens has a large focal length but small converging power. The small converging power of the thin-eye lens is sufficient to converge the parallel rays of light coming from a distant object to form an image on the retina.
(b) When the eye is focusing on a nearby object, it is said to be accommodated, as it becomes more convex.
To focus on nearby objects, the ciliary muscles of the eye contract due to which the suspensory ligaments get loose and stop pulling the eye lens and it becomes thick. The thick eye-lens has a small focal length but large converging power. The large converging power of the thick-eye lens helps in converging the diverging light rays coming from the nearby object to form an image on the retina.
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