Final answer:
The structure positioned behind the iris that focuses images onto the retina is the lens, which adjusts its shape to focus light for clear vision, with the aid of ciliary muscles and in coordination with the cornea.
Step-by-step explanation:
The structure positioned just behind the iris that serves to focus images onto the retina is the lens of the eye.
The lens is a transparent, convex structure that, in conjunction with the cornea, refracts light to concentrate it on the retina. The retina contains the photoreceptive cells essential for vision, particularly concentrated in the fovea for high acuity. The iris, the colored part of the eye, regulates the amount of light that enters through the pupil. The lens's ability to adjust its power enables it to produce clear images on the retina for objects at various distances, a process known as accommodation.
This remarkable adjustment is carried out by the ciliary muscles, which change the shape of the lens. When focusing on closer objects, the lens becomes rounder, while for distant objects, it flattens. Due to its crucial role in vision, any changes in the lens's clarity or flexibility, such as with cataracts or presbyopia, can significantly affect visual acuity.