The Eye 1

The eye

Layers of the eye:
1-Outer fibrous, sclera (opaque) and cornea (transparent).
2-Middle vascular layer formed of:
a- Choroid plexus.
b- Ciliary body consist of ciliary muscle and ciliary process.
c- Iris (separating anterior chamber from posterior chamber).
3- Inner sensitive layer called the retina

Optics of the Eye:

The lens system of the eye is composed of four refractive interfaces:
(1) The interface between air and the anterior surface of the cornea,
(2) The interface between the posterior surface of the cornea and the aqueous humor,
(3) The interface between the aqueous humor and the anterior surface of the lens
(4) The interface between the posterior surface of the lens and the vitreous humor.
The more a lens bends light rays, the greater is its “refractive power.”This refractive power is measured in terms of diopters
Reduced Eye:
If all the refractive surfaces of the eye are algebraically added together and then considered to be one single lens, the optics of the normal eye may be simplified and represented as a “reduced eye”. In the reduced eye, a single refractive surface is considered to exist, with its central point 17 millimeters in front of the retina and a total refractive power of 59 diopters when the lens is accommodated for distant vision. About two thirds of the 59 diopters of refractive power of the eye is provided by the anterior surface of the cornea (not by the eye lens). The principal reason for this is that the refractive index of the cornea is markedly different from that of air, while the refractive index of the eye lens is not greatly different from that of the aqueous humor and vitreous humor. The total refractive power of the internal lens of the eye is only 20 diopters ( about one third the total refractive power of the eye). But the importance of the internal lens is that, its curvature can be increased markedly to provide “accommodation,”
Mechanism of “Accommodation”
Accommodation: is the process by which the eye changes optical power to maintain a clear image of the object as its move closer to the eye.
Accommodation is an active process requiring muscular effort and therefore can be tiring.
In children, the refractive power of the lens of the eye can be increased voluntarily from 20 diopters to about 34 diopters; this in an “accommodation” of 14 diopters.
In a young person, the lens is composed of a strong elastic capsule filled with proteinaceous, but transparent fluid. About 70 ligaments attach radially around the lens, pulling the lens edges. At the attachments of the lens ligaments to the eyeball is the ciliary muscle.
The ciliary muscle is controlled by parasympathetic nerve transmitted to the eye through the third cranial nerve(Oculomotor) Under normal conditions of the eye The muscle is relaxed
The mechanism is as follows:when the eye focuses on near objects ,Stimulation of the parasympathetic nerves contracts of ciliary muscle fibers, which relaxes the lens ligaments, and the lens assumes a more spherical shape. the shape of the lens is changed from that of a moderately convex lens to that of a very convex lens. thus allowing the lens to increase its refractive power and the eye focuses on objects nearer to it .
In accommodation for near vision the lens increase its curvature converging of the eye and constriction of the pupils
(Note. Sympathetic stimulation has an additional effect in relaxing the ciliary muscle, but this effect is so weak and have no role in the normal accommodation mechanism)

Near point of the vision:

Is the nearest point to the eye at which an object can be seen clearly by accommodation. This near point recedes with advancing age from approximately 9cm at age of 10 year to approximately 83cm at age of 60 years
Presbyopia.
As a person grows older, the lens grows larger and thicker and becomes less elastic. The ability of the lens to change shape decreases with age. The power of accommodation decreases from about 14 diopters in a child to less than 2 diopters by the time a person reaches 45 to 50 years; it then decreases to essentially 0 diopters at age 70 years. Thereafter, the lens remains almost totally nonaccommodating, a condition known as “presbyopia.”
Errors of Refraction
Emmetropia (Normal Vision), The eye is considered to be normal, if parallel light rays from distant objects are focused on the retina when the ciliary muscle is completely relaxed. This means that the emmetropic eye can see all distant objects clearly with its ciliary muscle relaxed. However, to focus objects at close range, the eye must contract its ciliary muscle and thereby provide appropriate degrees of accommodation
Hyperopia (Farsightedness): Due to either an eyeball that is too short or, occasionally, a lens system that is too weak. In this condition, by the relaxed ciliary muscle parallel light rays are focused behind the retina. By using the mechanism of accommodation, a farsighted person is capable of seeing distant objects clearly (i.e the ciliary muscle must contract to increase the refractive power of the lens).
Correction of Hyperopia by the Use of a convex Lenses



Myopia (Nearsightedness) This is usually due to too long an eyeball, or too much refractive power in the lens system of the eye.
When the ciliary muscle is relaxed, the light rays coming from distant objects are focused in front of the retina
No mechanism exists by which the eye can decrease the strength of its lens to less than that which exists when the ciliary muscle is completely relaxed.
Correction of Myopia by Use of a concave spherical lens, which will diverge rays





Astigmatism
The cornea is not curved properly - the curve is irregular - usually one half is flatter than the other .
Astigmatism may also be caused by an irregularly shaped lens.
Here that light rays do not all come to a one focal point, in a blurred image.
The accommodative power of the eye can never compensate for astigmatism .
Correction of Astigmatism with a Cylindrical Lens.

Correction of Optical Abnormalities by Use of Contact Lenses

This is important in people whose eye refractive errors are caused by an abnormally shaped cornea such as those who have an odd-shaped, Without the contact lens, the bulging cornea causes such severe abnormality of vision that almost no glasses can correct the vision.
The contact lens has several advantages including
(1) the lens turns with the eye and gives a broader field of clear vision than glasses do
(2) the contact lens has little effect on the size of the object the person sees through the lens.