Final answer:
The transparent body behind the iris that focuses images on the retina is the crystalline lens, a convex structure that refracts light and works together with the iris to regulate light entry.
Step-by-step explanation:
The transparent body located behind the iris, which is responsible for focusing an image on the retina, is called the crystalline lens. This convex structure refracts (bends) light entering through the cornea and the pupil to focus on the photoreceptive cells of the retina. The iris regulates the amount of light that enters the eye by adjusting the size of the pupil. In high ambient light conditions, the iris contracts to shrink the pupil, while in low light conditions, it relaxes to enlarge the pupil, optimizing the amount of light that can enter and be focused by the lens onto the retina.