How the Eye Works
The cornea is the crystal clear dome that covers the front of the eye. The majority (70 percent) of the bending (refracting) of light rays is accomplished by the cornea. The shape of the cornea does not change (with the exception of small changes that occur over a lifetime.)
The crystalline lens finishes the focusing of light. It helps to "fine tune" vision, and it is able to change shape to allow focus on near objects. When in becomes cloudy, it is called a cataract.
Unlike the cornea, the lens can be made to change its shape (and therefore its refractive power) rapidly and voluntarily. Using its ability to change shape, the lens allows the eye to change its focal point. Changes in the shape of the lens will allow a normal eye to focus on near objects.
The iris is the part of the eye that gives it color (i.e., blue, green, brown). It functions like a shutter in a camera, allowing more or less light into the eye.
The pupil is the opening in the middle of the iris.
The retina is a thin layer of nerve tissue that senses light. Specialized cells called rods and cones convert light energy into nerve signals that travel through the optic nerve to the brain. The retina is analogous to the film in a camera.
The fovea is the center of the retina that receives the focus of the object of regard. Nerve cells are more densely packed in this area, so images that are focused on the fovea can be seen in greater detail.
The optic nerve runs from the eyeball to the brain. It carries information from the retina to the brain for interpretation.
The term "refractive" refers to the way light rays reflected from objects in our field of vision are brought into focus within the eye. The eye's complex optical components intercept, focus and process light into nerve impulses, which are sent to the brain, and that is how we "see."
The shape (curve) of the cornea, the power of the crystalline lens inside the eye, and the length of the eye together determine how rays of light are focused on the retina, the photosensitive membrane that lines the back wall of the eye. These three factors must be well matched in order for the eye to see clearly without correction (emmetropia).
If the cornea is too steeply curved or if the eye is too long, myopia (nearsightedness) results. If the cornea is too flat or the eye too short, hyperopia (farsightedness) results. In the picture above, light entering from the left travels through the cornea, then through the lens and is focused perfectly on the retina at the fovea.
In the normal (emmetropic) eye, distant objects are focused on the retina. To focus near objects on the retina, the lens changes its shape – that is, until the lens becomes stiff with age (presbyopia).