Human Eye is the most delicate and complicated natural optical instrument which enables us to see the wonderful world of light.
Structure of Human Eye
Diagram shows the section of a human eye by a horizontal plane. It is a spherical ball of diameter about 2.5 cm. Its essential parts are described below
Cornea : It is the front bulged out part of eye ball covered by transparent sclerotic.
Iris : It is the coloured region under cornea formed by choroid. Its colour differs from person to person.
Pupil : It is central circular aperture in the iris. Its normal diameter is 1 mm but it can contract in excess light and expand in dim light, by means of two sets of involuntary muscular fibres.
Crystalline lens : It is a double convex lens L immediately behind iris. This is made of transparent concentric layers whose optical density increases towards the centre of the lens.
Ciliary muscles : The lens is connected of the sclerotic by the ciliary muscles. These muscles change thickness of the lens by relaxing and exerting pressure.
Aqueous humour : Anterior chamber is filled with a transparent liquid of refractive index. The liquid is called the aqueous humour.
Vitreous humour : Posterior chamber is filled with a transparent watery liquid with little common salt having some refractive index. The liquid is called the vitreous humour.
Retina : It forms innermost coat in the interior of the eye. It consists of a thin membrane which is rich in nerve fibres, containing two kinds of vision cells called rods and cones and blood vessels. It is sensitive to light, for it is a continuation of the optic nerves. It serves the purpose of a sensitive screen for the reception of the image formed by the lens system of the eye.
[The rods are responsible for colour vision in dim light (Scotopic vision).
The cones are responsible for vision under ordinary day light (Photopic vision).
Blind spot : The blind spot B. It is the spot where the optic nerves enter the eye. It is also slightly raised and insensitive to light, because it is not covered with choroid and retina.
Working (Action of the eye)
The human eye is like a camera. Its lens system forms an image on a light-sensitive screen called the retina. Light enters the eye through a thin membrane called the cornea. It forms the transparent bulge on the front surface of the eyeball. The eyeball is approximately spherical in shape with a diameter of about 2.3 cm. Most of the refraction for the light rays entering the eye occurs at the outer surface of the cornea. The crystalline lens merely provides the finer adjustment of focal length required to focus objects at different distances on the retina. We find a structure called iris behind the cornea. Iris is a dark muscular diaphragm that controls the size of the pupil. The pupil regulates and controls the amount of light entering the eye. The eye lens forms an inverted real image of the object on the retina. The retina is a delicate membrane having enormous number of light-sensitive cells. The light-sensitive cells get activated upon illumination and generate electrical signals. These signals are sent to the brain via the optic nerves. The brain interprets these signals, and finally, processes the information so that we perceive objects as they are.