How We See

The eye is described as the window of the body. It is a direct outgrowth or extension of the brain. The human eye has been described to belong to a group of eyes in nature known generally as the "camera-type eyes". This classification is based on the contents and working of the eye as an optical system which work like a camera.

Anatomically, the eye resembles a structure consisting of segments of two unequal spheres. The bigger sphere is the sclera and is behind the smaller sphere known as the cornea. Basically, the sclera has a whitish appearance while the cornea is transparent. Within the eyeball is the anterior chamber, posterior chamber and vitreous chamber.
The anterior chamber is located directly behind the cornea but in front of the iris which is a colored circular structure with a central structure called the pupil and separates the anterior chamber from the posterior chamber which is behind the iris but in front of the crystalline lens. The vitreous chamber is located behind the lens and extends backwards to the optic cup.
The human eye is made up of four refractive media; two solid and two liquid jelly-like substances. The cornea and the lens make up the two(solid) refractive surfaces while the aqueous and vitreous humors are the liquid or jelly-like refractive media. As we would expect, all the refractive media of the eye are transparent. They allow light rays to pass or travel through them.
The cornea has the ability to focus light the most. It forms roughly one-sixth of the external eyeball. It is oval from the outer side and circular at the inner side. It is the first and most powerful  refractive media of the eye. Light coming from objects that we look at are focused by the cornea. After passing the cornea, the light then passes through the aqueous humour in the anterior and posterior chambers and are further focused but this time only slightly because the aqeous humour has a significantly less refracive power or focusing ability. After passing through the cornea and aqueous humour, the light rays then pass throuhg the crystalline lens.
The crystalline lens is a biconvex structure attached to the ciliary body by zonular fibers. The zonular fibres are quite flexible and their tension is controlled by the cilliary mussles. Contraction of the cilliary muscles will cause the tension of the zonular fibres to decrease causing a slight foward buldge of the crystalline lens which in turn, results to an increase its refractive power. When the cilliary muscles relax, the tension of the zonular fibres will increase and the crystalline lens will be stretched out. Stretching out of the crystalline lens causes it to have a less convex shape and consequently, a lower refractive power. These properties ensure that the crystalline lens is able to alter its focal lens to view objects at varying distances(far and near) just the same way a camera can be adjusted to capture images at different distances.
Finally, the light passes through the vitreous humour contained in the vitreous chamber behind the crystaline lens. Just like the aqeous humuour, the vitreous humour also has a relatively low refractive power.  It is jelly-like and bears great resemblance to the aqueous except that it is more viscous.
Finally, after passing through the four refractive media, the light finally hits the retina. The retina is regarded as the neural tunic of the eye. The process described above are just the physical aspects of the vision process. When light hits the retina, the electrical process kicks off. Cells called photo receptors are responsible for converting light into electrical impulses which are the transfered to the brain for processing via the visual pathway. There are to types of photo receptors; the rods and the cons. The rods are active under low illumination and are insensitive to colour while the cones are active under bright light and are used to provide colour vision. These photo receptors work together to ensure that we see well both during the day and at night.
When the light is finally processed into electrical signals the signals leave the eyeball with the nerve fibre layer of the retina which then joins the optic nerve. From the optic nerve, the signal continue through the optic chiasm where signals at the nasal fibres deccussate or cross over to opposite sides. This feature enhances our binocular vision.