Physics

Human Eye
It is the most delicate and complicated natural optical instrument which enables us to see the wonderful world of light.
STRUCTURE :

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 buldged out part of eye ball covered by transparent sclerotic.

Cornea of the eye-front view
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.