Final answer:
The lens of the eye thickens when the ciliary muscles contract. This increased thickness allows the lens to bend light more strongly and focus it onto the retina for clear vision of objects up close.
Step-by-step explanation:
The lens of the eye thickens when the ciliary muscles contract.
The ciliary muscle, which surrounds the lens, is responsible for changing the shape of the lens to focus on nearby objects. When the ciliary muscles contract, they exert tension on the lens, causing it to thicken.
This increased thickness allows the lens to bend light more strongly, focusing it onto the retina for clear vision of objects up close.