Eye Care - The Normal Eye1
To understand more about how your eyes work, here is a brief description of the main structures of the eye and how they function:
Lens
The lens is a transparent body behind the iris, the coloured part of the eye.
The lens bends light rays so that they form a clear image at the back of the
eye – on the retina. As the lens is elastic, it can change shape, getting
fatter to focus close objects and thinner for distant objects.
Cornea
The cornea is the transparent membrane which forms the outer coating at the
front of the eyeball and covers the iris and pupil. It also focuses light on
the retina.
Pupil
The pupil is the dark circular hole in the centre of the iris.
Iris
The iris is the coloured circle surrounding the pupil. It changes the size of
the pupil and allows different amounts of light to enter the eye.
Retina
The retina is the light-sensitive inner lining of the back of the eye. Imagine
that the eye is like a camera, and the retina is the film. Rays of light enter
the eye and are focused on the retina by the cornea and lens. The retina
produces an image which is sent along the optic nerve for the brain to
interpret, rather like developing a camera film.
Macula
The macula is the small area at the centre of the retina responsible for what we see straight in front of us, at the centre of our field of vision. The macula is very important as it gives us the vision needed for detailed activities such as reading and writing, and the ability to appreciate colour.
Sclera
The sclera is the white of the eye and forms the outer coating of the eyeball.
Conjunctiva
The conjunctiva is a mucous membrane that covers the exposed front portion of
the sclera and lines the inside of the eyelids.
Tear film
The tear film is the moist layer that covers and protects the outer surfaces at the front of the eye.
Vitreous
The vitreous is the clear, jelly-like substance that fills the inside of the eye from the lens to the retina.
How the Eye works:
The eyes do not actually see objects -- what they see is the light that objects reflect. When these light rays enter the eye, they are absorbed and converted into electrical signals by retinal nerves. These signals are subsequently sent to the brain where they are interpreted as visual images. In a normal eye, light rays enter the eye through the "cornea" (window of the eye) and are focused with the help of a "crystalline lens" behind the pupil at a point directly on the retina (the light sensitive nervous tissue at the back of the eyeball). However, only about four in ten people have normal visual acuity. For the rest, clear vision may be achieved by refocusing light rays with the use of corrective lenses.
How a contact lens works:
Contact lenses are delicately-crafted, very thin, small, optical discs, worn
directly on the eye. They are comfortably held in place by a natural layer of
tears present between the contact lens and the cornea. Contacts eliminate the
eyeglass barriers that interfere with the line of sight above, below and to the
sides of the eye, offering outstanding peripheral vision. In addition, contacts
can reduce or eliminate the image distortion sometimes caused by eyeglasses.
There are several types of contact lenses designed to fit the most common vision conditions. Following are brief descriptions of these conditions: