The eye is the organ of sight and is shaped as a slightly irregular hollow sphere. Various structures in the eye enable it to translate light into recognizable images. Among these are the cornea, the lens, and the retina.
Light first passes through the cornea, a clear dome-like structure covering the iris, or colored part, of the eye. The cornea bends, or refracts, the light onto the lens. The light is then refracted a second time while passing through the lens, finally focusing on the retina. The retina is the light sensitive part of the eye. Impulses travel down the optic nerve to the occipital lobe of the brain, which then interprets the image in the correct perspective.
The shape of the eye is very important in keeping the things we see in focus. If the shape of the eye changes, it affects a person’s vision.
Normally, light is precisely focused onto the retina at a location called the focal point. A nearsighted eye is longer from front to back than a normal eye causing light to be focused in front of the retina instead of directly onto it. This makes it difficult to see objects that are far away. Glasses with concave lenses are used to correct nearsightedness. The concave lens focuses light back onto the focal point of the retina.
Farsightedness occurs when the length of the eye is too short. Light is focused at a point behind the retina, making it difficult to see objects that are up close. A convex lens is used to correct farsightedness because it directs the focal point back onto the retina.
Update Date 10/27/2014
Updated by: Laura J. Martin, MD, MPH, ABIM Board Certified in Internal Medicine and Hospice and Palliative Medicine, Atlanta, GA. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.