Light, refraction and its importance.
Light entering the eye is first bent, or refracted, by the cornea — the clear window on the outer front surface of the eyeball. The cornea provides most of the eye’s optical power or light-bending ability.
After the light passes through the cornea, it is bent again — to a more finely adjusted focus — by the crystalline lens inside the eye. The lens focuses the light on the retina. This is achieved by the ciliary muscles in the eye changing the shape of the lens, bending or flattening it to focus the light rays on the retina.
This adjustment in the lens, known as accommodation, is necessary for bringing near and far objects into focus. The process of bending light to produce a focused image on the retina is called “refraction”. Ideally, the light is “refracted,” or redirected, in such a manner that the rays are focused into a precise image on the retina.
Most vision problems occur because of an error in how our eyes refract light. In nearsightedness (myopia), the light rays form an image in front of the retina. In farsightedness (hypermetropia), the rays focus behind the retina. In astigmatism, the curvature of the cornea is irregular, causing light rays to focus to more than one place so that a single clear image cannot be formed on the retina, resulting in blurred vision. As we age, we find reading or performing close-up activities more difficult. This condition is called presbyopia, and results from the crystalline lens being less flexible, and therefore less able to bend light.
Since changing the apparent refraction of the eye is relatively easy through the use of corrective spectacle or contact lenses, many of the conditions that contribute to unclear vision can be readily corrected.