Final answer:
The lenses in the boy's eyes thicken and increase in curvature as he moves his finger closer, due to the ciliary muscles contracting, a process called accommodation. This change in the lens shape is necessary for the eye to focus light precisely on the retina for clear vision at different distances.
Step-by-step explanation:
As the boy stretches out his arm and focuses on his finger, then moves it closer to just a few centimeters from his eyes, the lenses in his eyes undergo a change in shape to maintain focus. This process, known as accommodation, involves the eye adjusting its focal length and power. The ciliary muscles around the lens contract to increase the lens's curvature for closer objects. This contraction thickens the lens, allowing it to bend light rays more sharply and focus the light more directly on the retina, regardless of the distance.
For objects at a distance, like when the finger is at arm's length, the ciliary muscles relax, causing the lens to flatten and reducing its focusing power, which is necessary for focusing on far-away objects. In comparison, for up-close viewing, the need for greater focusing power is met by the increased curvature of the lens when the ciliary muscles contract. This adaptation enables the formation of a clear image on the retina whether we are looking at objects that are close or at a distance.