The eye

• A sensory organ that facilitates sight• It allows light that passed over objects to

enter in specific quantities into it• As the light enters the eye it is refracted (bent)

as it travels from one media to another ( the cornea, through the aqueous humour, through the lens, through the vitreous humour and onto the retina)

• The image formson the retina

The eye is made of three coats, or tunics.1. The outermost coat consists of the cornea and the sclera; 2. The middle coat contains the main blood supply to the eye and

consists of the choroid, the ciliary body, and the iris. 3. The innermost layer is the retina. • The sclera, or the white of the eye, is composed of tough fibrous

tissue. (it is coved by Conjunctiva, a mucous membrane that keeps the eye from getting dry)

• The cornea, a part of the sclera, is the transparent window of the eye through which light passes.

• Behind the cornea is a watery fluid called the aqueous humor• Behind the aqueous humor is a colored ring called the iris. The

color of the iris is inherited and does not affect vision. The iris is like a muscular curtain that opens and closes. It controls the amount of light entering the eye through the pupil, an opening in the iris.

• The pupil looks like a black spot.

• The choroid is a layer of blood vessels and connective tissue squeezed between the sclera and the retina. It supplies nutrients to the eye.

• The ciliary body is a muscular structure that holds the lens in place and changes its shape.

• The lens is a colorless, nearly transparent double convex structure, similar to an ordinary magnifying glass. Its only function is to focus light rays onto the retina.

• The ability of the lens to adjust from a distant to a near focus is called Accommodation. By contracting, the ciliary muscle pushes the lens to make it thicker in the middle. By relaxing, the muscle pulls the lens and flattens it. To see objects clearly when they are close to the eyes the lens is squeezed together and thickened. To see distant objects clearly it is flattened.

• The retina is a soft, transparent layer of nervous tissue made up of millions of light receptors. The image is formed here

Complete the table below

The eye is like a cameraPart of eye Its purpose in the eye Matching part of

cameraPurpose in camera

Pupil The small hole through which light enters

Iris Controls the amount of light entering the eye

Lens Focuses image on retina

Ciliary muscles Controls the thickness of the lens

Choroid Supplies nutrition to the eye structures

Retina Image formed on this layer

Optic nerve Transmits the image to the brain

• Vitamin A, also called retinol, is a fat-soluble vitamin that is readily destroyed upon exposure to heat, light, or air.

• The vitamin has a direct role in vision and is a component of a pigment present in the retina of the eye. It is essential for the proper functioning of most body organs and also affects the functioning of the immune system.

• Vitamin A deficiency results in various disorders that most commonly involve the eye and the epithelial tissues the skin and the mucous membranes lining the internal body surfaces.

• An early symptom of vitamin A deficiency is the development of night blindness, and continued deficiency eventually results in loss of sight.

• If deficiency is prolonged, the skin may become dry and rough. Vitamin A deficiency may also result in defective bone and teeth formation.

• Excessive intake of vitamin A causes a toxic condition. The symptoms may include nausea, coarsening and loss of hair, drying and scaling of the skin, bone pain, fatigue, and drowsiness. There may also be blurred vision and headache in adults, and growth failure, enlargement of the liver, and nervous irritability in children

Long sightedness

• Long sightedness, also called farsightedness and hyperopia, is a vision problem in which an individual can see clearly objects a long distance away, but those up close are blurry.

• Long sightedness is usually present at birth and is hereditary

• Often when the eyeball is too short or the lens cannot become round enough

Various eye care professionals are involved in the treatment and management of hyperopia. At the conclusion of an eye examination, an eye doctor may provide the patient with an eyeglass

prescription for corrective lenses.Minor amounts of hyperopia are sometimes left uncorrected. However, larger amounts may be

corrected with convex lenses in eyeglasses or contact lenses..Hyperopia is sometimes correctable with various refractive surgery procedures (LASIK).

Below is correction using convex lens.

Short sightedness

• Short sightedness, also called nearsightedness and Myopia, is a vision problem in which an individual can see clearly objects a short distance away, but those far away are blurry.

• Most cases of shortsightedness are caused by a change in the eyeball's natural shape. The eyeball normally being round, appears to be an oval shape in shortsighted people. Shortsightedness can also be a cause of changes in the cornea lens. The unnatural changes in the eyes cause light rays that enter them to focus on the front of the retina rather than on the retina directly.

• H/W: Research on prevention/correction of Myopia