The cornea at the front of the eye is the most important structure for focusing light onto the back of the eye (the retina). The crystalline lens in the eye aids in this process by altering its shape to change the focus of the eye from distance to near.
In a normal or emmetropic eye, light rays from a distant source are sharply focused by the cornea and lens onto the retina, resulting in clear distance vision. Emmetropia is achieved when there is a balance between the optical power and the length of the eye, allowing images to be perfectly focused on the retina.
Refractive errors such as myopia (short-sightedness), hyperopia (long-sightedness) and astigmatism occur when the cornea’s optical power and length of the eye are not aligned, thereby preventing light rays from focusing accurately onto the retina.