The light rays of the image enter the eye through the cornea, a clear window similar to a lens filter, which provides most of the focusing power of the eye.
The cornea is composed of several layers of tissue.
The outer layer or epithelium is the eye’s protective layer. This layer is made up of cells that have the ability to grow back within five to seven days, and therefore, allow for fast healing of superficial injuries. Most of the inner layers provide strength to the eye.
The middle inner layer, the stroma, is the largest layer and the part of the cornea that is typically modified in refractive surgery to change the focus. Damage to this layer and posterior in the cornea will result in corneal scarring.
The last layer is the endothelium, a very important layer that is largely responsible for keeping the cornea clear by regulating solute transport.
After the cornea, the partially focused image then travels through the pupil. the pupil is the “black circle” that you see in people’s eyes.
The iris, the colored part of the eye (ie. blue, green, brown, or hazel) determines the size of the pupil. The primary function of the iris is to control the size of the pupil and therefore the amount of light entering the eye. This is achieved through contraction or expansion of the muscles of the iris.
When you are in a bright environment, the iris contracts to allow less light through and control optical aberrations. when it is dark, the pupil expands to allow more light to reach the back of the eye. This is why people notice more night vision blur.
The lens, the next element in this optical system, is a clear structure located just behind the pupil.
Its primary function is to provide fine-tuning for focusing and reading. the lens performs this function by altering its shape.
At about the age of 40-50, the lens becomes less flexible as presbyopia sets in. Presbyopia, or loss of near vision is why many of us who never had to wear glasses before, need them to read with after forty years of age.
Finally, sometime around age 60 to 70, the lens becomes cloudy and hard (cataract formation), preventing light from entering the eye. These cataracts may then be removed with advanced techniques.
The lens fine-tunes the image to focus it properly on the retina.
The retina is a thin layer of nerve tissue that lines the inside of the eye and functions like the film in a camera. The retina transforms the image into electric impulses that are then carried by the optic nerve to the brain transforms the light.
For you to see clearly, light must be focused precisely on the retina.
Glasses or contact lenses are required when your eye cannot focus light properly.
Damage to the photoreceptors in the retina is irreversible. This is why it is important to protect your eyes from external factors. Wear sunglasses to protect your eyes from UV light. Patients who have diabetes should get an annual eye exam since diabetes is the leading cause of blindness in the U.S.
If you can see objects nearby with no problem, but reading road signs or making out the writing on the board at school is more difficult, you may be near- or shortsighted.
If you experience a distortion or blurring of images at all distances — nearby as well as far — you may have astigmatism. Even if your vision is fairly sharp, headache, fatigue, squinting and eye discomfort or irritation may indicate a slight degree of astigmatism.