What part of the eye flexes to focus light onto the back of the retina?

The lens is the part of the eye that flexes to focus light onto the back of the retina. It changes shape depending on the distance of the object being observed. When looking at something nearby, the lens becomes thicker (more convex) to bend the incoming light rays more sharply, allowing them to converge on the retina. Conversely, when focusing on distant objects, the lens flattens out. This ability to change shape is crucial for clear vision at various distances.

The cornea, while it does play a significant role in focusing light, has a fixed shape and does not flex. The iris is responsible for controlling the size of the pupil and regulating the amount of light that enters the eye but does not focus light itself. The pupil is simply the opening that adjusts in size based on light conditions and does not perform any focusing function. Understanding the distinct functions of these different parts of the eye helps clarify why the lens is the critical component for focusing light onto the retina.