The Human Eye

• The human eye is an organ that reacts in light and allows light perception, colour vision, and depth perception. A structure inside the eye called the cornea focuses light onto a light-sensitive membrane called the retina, which is similar to a camera lens that focuses light onto film. Engrained in the retina are millions of light-sensitive cells, known as rods and cones.

Rods enable the monochrome vision required in poor light, while cones enable colour vision and the detection of fine detail.

The human eye consists of three layers :

1. Outer Layer – consisting of the cornea and the sclera.
2. Middle Layer – consisting of the choroid, ciliary body, and iris.
3. Inner Layer – the retina, which is visible with the help of an instrument called the ophthalmoscope.

The cornea, which is also a part of the outer layer is a clear transparent membrane found in the front of the eye. It bends the light rays and helps them pass freely through the pupil. The sclera, which is the outermost layer surrounding the eyeball, gives most of the eye its white colour.

The middle layer between the retina and cornea is called the choroid. It contains blood vessels that supply the retina with nutrients and oxygen and remove its waste products.

Behind the cornea is a colourful, ring-shaped membrane called the iris. In the centre of the iris is the pupil, the black dot that changes size. It is an adjustable opening which can expand or contract to control the intensity of light.

Situated right behind the pupil is a colourless, transparent structure called the crystalline lens. Ciliary muscles surround the crystalline lens. These muscles hold the lens in position and also play an essential role in regulating vision.

The function of these muscles is when they relax, they pull on and flatten the lens, allowing the eye to see far away objects. To see nearer objects clearly, the ciliary muscle contracts to thicken the lens.

Two-thirds of the energy consumed by the eye is provided by the cornea while the remaining by the lens. The lens after receiving light through pupil then focuses it through the interior chamber of the eyeball, filled with a clear fluid like tissue called the vitreous humor.

After passing through the lens, light must travel through this humor before striking the sensitive layer of cells called the retina. When the image finally reaches the retina, it transforms this image into electrical impulses that are transported by the optic nerve fibres to the brain. The brain then translates these electrical impulses received by optic nerve into the images which a person sees.