the normal eye
The cornea is the clear window at the front of the eye, providing about two-thirds of its focusing power. Along with the natural lens, the cornea focuses light onto the retina, allowing us to see. The crystalline lens helps in this process by changing its shape to adjust focus from distance to near objects.
In a normal or emmetropic eye, light rays from a distant source are sharply focused by the cornea and lens directly onto the retina, resulting in clear distance vision. Emmetropia is achieved when the optical power and length of the eye are perfectly balanced, allowing images to be clearly focused on the retina. The lens can change shape to adjust focus for near objects. However, as people age, they lose the ability to focus on near objects and typically need reading glasses around age 50-55; this condition is known as presbyopia.
Refractive errors, such as myopia (short-sightedness), hyperopia (long-sightedness), and astigmatism, occur when the cornea’s optical power and the eye’s length are not aligned, preventing light rays from focusing accurately on the retina.