Notes for human eye

1. The Human Eye It is a natural optical instrument which is used to see the objects by human beings. It is like a camera which has lens and screen system.

(i) Retina : It is a light sensitive screen inside the eye on which image is formed. It contains rods and cones.

(ii) Cornea : It is a thin membrane which covers the eye ball. It acts like a lens which refracts the light entering the eye.

(iii) Aqueous humour : It is fluid which fills the space between cornea and eye lens.

(iv) Eye lens : It is a Convex lens made of transparent and flexible jelly like material. Its curvature can be adjusted with the help of ciliary muscles.

(v) Pupil : It is a hole in the middle of iris through which light enters the eye. It appears black because light falling on it goes into the eye and does not come back.

(vi) Ciliary muscles : These are the muscles which are attached to eye lens and can modify the shape of eye lens which leads to the variation in focal lengths.

(vii) Iris : It controls the amount of light entering the eye by changing the size of pupil.

(viii) Optical nerve : These are the nerves which take the image to the brain in the form of electrical signals.

2. Accomodation power : The ability of eye to change the focal length of eye lens with the help of ciliary muscles to get the clear view of nearby objects (about 25 cm) and far distant objects (at infinity).

3. Colour blindness : Some people do not possess some cone cells that respond to certain specific colours due to genetic disorder.

4. Myopia (Short sightedness) : It is a kind of defect in human eye due to which a person can see near objects clearly but he can not see the distant objects clearly. Myopia is due to

(i) excessive curvature of cornea.
(ii) elongation of eye ball.