The human eye is one of the most important valuable sense organ, which the nature has given us.

The main parts of the human eye are:
a) Cornea : Front part of the eye bulging outwards.
b) Iris : It controls the amount of light entering the eye, by varying the size of pupil.
c) Pupil : It is the central hole in the iris.
d) Ciliary muscles : These muscles hold the eye in position.
e) Eye lens : It is a crystalline lens made of transparent material.
f) Aqueous humour : Liquid filling the space between cornea and eye lens.
g) Retina : It is light sensitive screen on which image is formed.
h) Vitreous humour : It is transparent liquid filling the space between eye lens and retina.
i) Optic nerves : These carry electrical signals to the brain.
j) Blind spot : A raised spot on the retina which is insensitive to light.
k) Optic axis : Line passing through the centre of the eye lens and cornea

Accommodation : The ability of the eye lens to adjust its focal length according to the distance of object from the eye is called accommodation.

Amount of light entering the eye is controlled by changing the size of pupil. When light is dim, it opens us completely, and when it is very bright it becomes very small.

Least distance of distant vison or near point : It is the minimum distance at which objects can bee seen clearly. For normal vision it is 25 cm, it is represented by D.

Far point : The farthest distance up to which eye can see clearly is called far point, for normal vision far point is infinity. With age, eye lens loses flexibility and its accommodation decreases.

Range of vision : It is minimum to maximum distance, up to which eye can see clearly. It is 25 cm to infinity.

On a camera, image formed is permanent but on the eye it remains only for 1/6th of a second.

Persistence of vision : The continuance of the impression of the object on the eye is called persistence of vision.

If the sequence of still pictures are moved at the rate of 24 or more per second, these seem to merge into one another, giving impression of continuity. This principle is used in cinematography.

Power of accommodation : It is the maximum variation of power of eye.

Cataract : Opaque membrane formed on the eye is called cataract, it can be removed by surgery.

Phorias : It is the defect when two images of an object are seen simultaneously.

Our eyes have a large number of light sensitive cells in the shapes of rods and cones. The rod-shaped cells respond to intensity of light and cone-shaped to different types of colours.

There are mainly four types of eye defects:
a) Myopia or near-sightedness : In myopia a person can see nearby objects but distant objects cannot be seen clearly.
i) Cause of myopia : Either cornea has excessive curvature or elongation of the eye-ball.
ii) Remedy : Using suitable diverging (concave) lens.

b) Hypermetropia or far-sightedness : In this case, a person can see far-off objects clearly but objects lying nearby cannot be seen properly.
i) Cause of hypermetropia : Either focal length of eye lens is too large or eye-ball has become too short.
ii) Remedy : Using suitable converging (convex) lens.

c) Presbyopia : In this case a person suffers from both myopia and hypermetropia.
i) Cause of presbyopia : Gradual weakening of ciliary muscles and diminishing flexibility of eye lens.
ii) Remedy : Suitable bifocal lenses.

d) Astigmatism : A person suffering from astigmatism cannot focus on horizontal and vertical lines simultaneously.
i) Cause of astigmatism : Cornea not being perfectly spherical .
ii) Remedy : Using suitable cylindrical lenses.

Splitting of white light into colours is called dispersion of light.

Dispersion of light occurs as different colours have different speeds in a transparent medium.

The pattern of seven colours is called spectrum.

Red, blue and green are primary colours as by mixing these various other colours can be obtained.

Colours obtained by mixing primary colours are called composite or compound colors. For example: magenta, cyan and yellow are composite colours.

Colours which on mixing give white colour are called complementary colours.

Painter obtains different shades by subtractive mixing of coloured pigments.

An object appears to be of that particular colour in white light, the colour which it reflects, other colours are absorbed by it.