Members of the animal kingdom use different strategies to detect light and focus it to form images. Human eyes are "camera-type eyes," which means they work like camera lenses focusing light onto film. The cornea and lens of the eye are analogous to the camera lens, while the retina of the eye is like the film.
Eye Structure and Function
To understand how the eye sees, it helps to know the eye structures and functions:
The Retina and the Optic Nerve
The coating on the interior back of the eye is called the retina. When light strikes the retina, two types of cells are activated. Rods detect light and dark and help form images under dim conditions. Cones are responsible for color vision. The three types of cones are called red, green, and blue, but each actually detects a range of wavelengths and not these specific colors. When you focus clearly on an object, light strikes a region called the fovea. The fovea is packed with cones and allows sharp vision. Rods outside the fovea are largely responsible for peripheral vision.
Rods and cones convert light into an electric signal that is carried from the optic nerve to the brain. The brain translates nerve impulses to form an image. Three-dimensional information comes from comparing the differences between the images formed by each eye.
Common Vision Problems
The most common vision problems are myopia (nearsightedness), hyperopia(farsightedness), presbyopia (age-related farsightedness), and astigmatism. Astigmatism results when the curvature of the eye isn't truly spherical, so light is focused unevenly. Myopia and hyperopia occur when the eye is too narrow or too wide to focus light onto the retina. In nearsightedness, the focal point is before the retina; in farsightedness, it is past the retina. In presbyopia, the lens is stiffened so it's hard to bring close objects into focus.
Other eye problems include glaucoma (increased fluid pressure, which can damage the optic nerve), cataracts (clouding and hardening of the lens), and macular degeneration (degeneration of the retina).
Weird Eye Facts
The functioning of the eye is fairly simple, but there are some details you might not know:
By Anne Marie Helmenstine
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