Final answer:
The contraction of ciliary muscles allows the lens to become thicker, which is true. This process helps in focusing light onto the retina for clear vision at various distances, a process known as accommodation.
Step-by-step explanation:
The statement that the contraction of the ciliary muscles allows the lens to become thicker is true. In the human eye, the process known as accommodation involves the changing of the lens shape to focus light properly onto the retina. When looking at objects up close, the ciliary muscles contract, causing the lens to become more spherical or thicker, which increases its converging power to bend light more effectively. Conversely, for distant vision, the ciliary muscles relax, and the lens flattens out, becoming less powerful. This ability of the lens to adjust its shape and thus its focal length ensures that images are focused onto the retina whether they come from near or far objects.