When a ciliary muscles contracts, the eye lens becomes __________.
Thicker and the focal length decreases
The ciliary muscles are attached to the eye lens through the suspensory ligaments. When these muscles contract, they release the tension on the ligaments, and the elastic lens is free to bulge into a more rounded, thicker shape.
A thicker (more strongly curved) lens has more converging power, so its focal length decreases. This is exactly what the eye needs to focus on nearby objects. If the muscles relaxed the lens would become thinner and the focal length would increase, so only the thicker-lens/shorter-focal-length combination matches contraction.
Hence, the answer is Thicker and the focal length decreases.
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