The main parts of the human eye are: Cornea, Iris, Pupil, Ciliary muscles, Eye lens, Ratena and Optic nerve. The eye -ball is approximately spherical in shape having a diameter of about 2.5 cm. We will now describe the construction and working of the eye.
Construction of the Eye
The front of the eyes is called cornea. It is made of a transparent substance and it is bulging outward. The outer surface of cornea is convex in shape. The light coming from objects enters the eye through cornea. Just behind the cornea is the iris (or coloured diaphragm).
Iris is a flat, coloured, ring- shaped membrane behind the cornea of the eye. There is a hole in the middle of the iris which is called pupil of the eye. Thus, pupil is a hole in the middle of the iris. The pupil appears black because no light is reflected from it.
The eye- lens is a convex lens made of a transparent, soft and flexible material like a jelly made of proteins. Being flexible, the eye- lens can change its shape (it can become thin or thick) to focus light on to the retina. The eye- lens is held in position by suspensory ligaments.
One end of suspensory ligaments is attached to the eye- lens and their others end is attached to ciliary muscles.
Ciliary muscles change the thickness of eye- lens while focusing. In other words, the focal length of eye- lens (and hence its converging power) can be changed by changing its shape by the action of Ciliary muscles.
In this respect an eye differs from a camera. The focal length of the convex lens used in a camera is fixed and cannot be changed but the focal length by the action Ciliary musles.
The scree on which the image is formed in the eye is called retina. The retina is behind thy eye- lens and at the back part of the eye. The retina of an eye is just like the film in a camera.
The retina is a delicate membrane having large number of light sensitivitive Cells called 'rods' and 'colour of objects' respectively, by generating electrical signals. At the junction of optic nerve and retina in the eye 👁️, there are no light sensitive cells (no rods or cones) due to which no vision is possible at that spot. This is called blind spot.
Thus, blind spot is a small area of the retina insensitive to light where the optic nerve leaves the eye. When the image of an object falls on the blind spot, it cannot be seen by the eye. It should be noted that there is an eye- lid in front of the eye- lid is closed, no light enters the eye. The space between cornea and eye- lens is filled with a watery liquid called 'aqueous humour'. And the space between eye- lens and retina is filled with a transparent jelly like substances called 'vitreous humour' which supports the back of the eye.
