Final answer:
The lens is responsible for focusing light waves onto the retina, working with the cornea to form a clear image that can be detected by photoreceptor cells.
Step-by-step explanation:
The structure responsible for focusing light waves onto the retina is the lens. The cornea and the lens of the eye act together to form a real image on the light-sensing retina. The lens is a clear, curved structure that, along with the cornea, refracts (bends) light to focus the image on the retina. This focusing process is vital for vision: the lens must change shape to bend light by the appropriate amount to ensure that images are brought into focus, whether they are near or far. The retina, at the back of the eye, contains photoreceptor cells known as rods and cones which detect the focused light, with signals then being sent to the brain via the optic nerve.