.......... can alter the curvature and thickness of the lens.
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Answer:
The eye lens is made up of a fibrous, jelly-like material. Its curvature can be modified to change its focal length to some extent by the ciliary muscles. When the muscles are relaxed, the curvature decreases making the lens thin and the focal length increases.
The crystalline lens provides approximately one-third of the focusing power of the eye. The lens is flexible and its curvature can change by influence of the ciliary body. When it changes curvature, the lens gets thicker and the power increases so that the eye can focus on images at different distances. This change in focusing is called accommodation. When our eye looks at something at a very close distance to us, our ciliary body contracts and this loosens the lens zonules which hold the lens in place and the lens thickens.
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