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Adjusting the focal length of the eye lens to view objects at different distances is done by:
Topic: Accommodation — ciliary muscles
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Answer
(c) Ciliary muscles.
This is ACCOMMODATION, and it is performed by the CILIARY MUSCLES, which alter the curvature of the lens: for a NEAR object the ciliary muscles CONTRACT, the suspensory ligaments slacken, and the lens becomes MORE CONVEX (thicker), bending light more strongly for a DISTANT object the muscles RELAX, the ligaments tighten, and the lens is pulled THINNER
THE TRAP is the IRIS. The iris also adjusts, but it controls the SIZE OF THE PUPIL and therefore how much LIGHT enters — not the focal length. Read what is being adjusted.
The cornea and sclera are fixed structures and cannot change shape at all: the cornea is the transparent front of the eye, the sclera its tough white outer coat.
It is the gradual loss of this power of accommodation with age that causes PRESBYOPIA.
Ciliary muscles.