Final answer:
The flexible structure is the lens, which changes its shape to focus images on the retina, enabling clear vision at different distances.
Step-by-step explanation:
The flexible structure that changes in thickness and curvature to bring images into focus on the retina is the lens. The lens, along with the cornea, acts to form a real image on the light-sensitive retina. The retina requires a precise focal length that is adjusted by the change in shape of the lens, regulated by ciliary muscles. This adjustment allows for clear vision at varying distances. The cornea serves as a protective layer and assists in focusing, while the iris adjusts the amount of light entering the eye through the pupil. It is the flexible lens that changes its power to provide a focused image for different object distances.