WORKING OF EYE

* The eye is a natural optical instrument which enables us to see things around us.

* The eye consist eyeball which is spherical in shape fitted in the eye socket.

* CORNEA: The front part of the eyeball is cornea.

a) It is made up of transparent material through which light enters the eye.

b) It protects the eye from dirt, dust and small insect that enters in our eye.

c) 70% of the focusing of light falling on the eye is done by the cornea.

* IRIS AND PUPIL: Behind the cornea is the iris with a hole in the centre called pupil.

PUPIL: Pupil is the dark circle seen in the centre of the eye.

a) Light goes into the eye through the pupil.

b) The size of the pupil is controlled by the iris.

IRIS: Iris is a thin circular structure.

a) It works like the shutter of a camera.

b) It controls the amount of light entering the eye by increasing and decreasing the size of the pupil: In bright light, iris expands and reduce the size of pupil so that right amount of light enters in eye.

In dull light, iris contracts to increase the size of the pupil to admit more light.

c) The colour of iris imparts distinct colour to the eyes of an individual. A person with blue iris has blue eyes.

AQUEOUS HUMOUR: The region between the cornea and the lens is filled with a transparent liquid called aqueous humour.

a) It is a clear, watery fluid produced by ciliary muscles. It supplies nutrients that nourishes the cornea and lens.

b) It also helps to maintain the convex shape of the cornea.

LENS: Lens is present behind the pupil which is made of a jelly like substance.

a) It is converging( Convex) lens which helps the parallel light rays to converge (meet) at one point (at the retina).

b) The lens is held by ciliary muscles, which can change the thickness of the lens and hence the focal length( length between the focus and optical centre), where focus is the point where all light rays meet.

c) Change in lens when seeing nearby objects: The ciliary muscles contract, make the lens thicker which reduce the focal length of the lens and the converging power of the lens increases and it can focus nearby objects clearly on retina.

d) Change in lens when seeing distant objects: The ciliary muscles expand, make the lens thinner which increase the focal length of the lens and the converging power of the lens decrease and it can focus distant objects clearly on retina.

VITREOUS HUMOUR: The region between the lens and retina is filled with a transparent jelly like substance called the vitreous humour.

a) It forms the major bulk of the eyeball.

b) It maintains the spherical shape of the eyeball.

c) It also helps to refracts the light onto the retina.

RETINA: It is the screen at the back of the eye on which the focused image of the object is formed.

a) The retina contains several nerve cells which sense the image formed on the retina.

b) Nerve cells are of two types:

Cone cells: They are sensitive to bright light means when you see the objects in the bright light, cone cells sense the image of the object which is formed on retina in bright light.

* They also help to detect the colour of the object.

Rod cells: They are sensitive to dim light means when you see the object in dim light, the image of the object which is formed on the retina in dim light is sensed by rod cells. The bats and owls are able to see the objects in dark due to presence of large numbers of rod cells in their eyes.

c) The image of an object formed on the retina is smaller than the object and is inverted(ulta).

OPTIC NERVES:

a) The optic nerve emerges from the back of the eyeball and goes to the brain.

b) The image sensed by the nerve cells are then transmitted to the brain by optic nerves.

c) Brain interprets the image as erect and of the correct size.

BLIND SPOT: The junction of the optic nerve and retina.

a) There are no photosensitive (nerve cells) cells at the spot, and hence no image is formed at this spot on the tretina.