In order for our eyes to see clearly, the rays coming into the eye must be refracted and focused on the surface at the back of the eye called the retina. This refraction of light is achieved by the cornea (the outermost transparent layer of the eye) and the lens. If the eye can focus rays coming from different distances on the retina, the refractive function of the eye is considered normal and this situation is called emmetropia. The eye consists of complementary structures and needs to be checked periodically due to its sensitive nature.
Eyeball: Eye is spherical with an average diameter of 2.5 cm. At the outermost layer, in the middle of the white sclera, there is the cornea in the shape of a watch glass. The uvea, which is the middle layer, consists of 3 parts: in the front, the iris, which gives the color of the eye; in the middle, the corpus ciliare; in the behind, the choroidea, which provides nutrition for most of the eye. The innermost layer is the retina, which is responsible for the first step of vision. A two-layered, transparent and slimy membrane called the conjunctiva surrounds the sclera. Between the cornea and the iris, there are chambers one of which is called the anterior chamber, which is 2.5 mm deep, filled with a transparent liquid called aqueous humor, and the other one called the posterior chamber, which is also filled with aqueous humor, between the iris and the intraocular lens, which we call the lens. The transparent vitreous behind the lens fills 3/4 of the eye, giving the sphere its shape. At the very back is the optic nerve head, which will transmit vision from the retina to the brain.
Eyelid: It is the part that protects the eye from the external environment. The muscles in the structure of the lids provide periodic movement of the lid with the clipping reflex. The tears secreted from the lacrimal gland act as a wiper with the movement of the lids, preventing the drying of the layers in the front of the eye and ensuring their cleaning.
Cornea: It is the anteriormost, convex, veinless layer of the eye, the part where the contact lens is placed. It is about half a millimeter thick and 12 mm in diameter. The cornea provides the most refraction of light rays entering the eye so that a clear image is formed on the retina (nerve layer). When the cornea refracts light rays less, it causes hyperopia; if it refracts much, it causes myopia; and if it does not refract it equally in all directions, it causes a refractive error called astigmatism.
Iris: It is the vascular layer that gives the eye its color. The contraction and relaxation of the muscles in its structure allows the pupil, which has a gap in the middle, to grow and shrink. It balances the light rays entering the eye by shrinking in the light and growing in the dark.
Lens: It is a transparent, veinless structure with a thickness of 5 mm and a diameter of 9 mm behind the iris. Its task is to refract light rays entering the eye, being the second layer to do that after the cornea. It differs from the cornea in that it has the flexibility and refraction ability of the lens, that is, the zoom feature, so that we can see the object at all distances, far and near.
Vitreus: It is a gel-like substance that fills the entire eye cavity behind the lens.
Retina: It is the nerve layer that covers the inner wall of the eye. It converts the image formed by the focusing of the light rays entering the eye into electrical signals and transmits them to the optic nerve.
The optic nerve (eye nerve): Nerve fibers coming out of the retina layer converge at one point in the eye and continue from the eye to the brain.