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The Hyman Eye and the Colourful World
In this chapter we will study Human eye that uses the light and enable us to see the objects. We will also use the idea of refraction of light in some optical phenomena in nature i.e. Rainbow formation, twinkling of star, blue and red colour of sky etc. Human Eye : A Sensitive sense organ It acts like a camera, enable us to capture the colourful picture of the surroundings. It forms an inverted, real image on light sensitive surface Retina

The Various parts of eye and their functions 1. Cornea : It is a thin membrane through which light enters. It forms the transparent bulge on the front of eyeball. Most of the refraction occurs at the outer surface of the cornea. Eyeball : it is approximately spherical in shape, with a diameter of about 2.3cm. Iris : It is a dark muscular diaphragm that controls the size of pupil. It is behind the cornea. Pupil : It regulates and control the amount of light entering the eye. It is the black opening between aqueous humour & lens. Crystalline eye lens : Provide the focussed real & inverted image of the object on the retina. It is composed of a fibrous, jelly like material. This is convex lens that converges light at retina. 114 X-Science

2. 3. 4. 5.


Ciliary muscles : It helps to change the curvature of eyelens and hence changes its focal length so that we can see the object clearly placed at different positon. Retina : Thin membrane with large no. of sensitive cells. When image formed at retina, light sensitive cells gets activated and generate electrical signal. These signals are sent to brain via optic nerue. Brain analyse these signals after which we perceive object as they are.

7. 8.

How pupil works ? Example : You would have observed that when you come out of the cinema hall after watching movie in the bright sun light, your eyes get closed . And when you entered the hall from the bright light, you won't be able to see and after some time you would be able to see. Here the pupil of an eye provide a variable aperture, whose size is controlled by iris a) b) When the light is bright : Iris contracts the pupil, so that less light enters the eye. When the light is din : Iris expand the pupil, so that more light enters the eye.

Pupil open completely, when iris is relaxed. Persistence of Vision : It is the time for which the sensation of an object continue th in the eye. It is about 1/16 of a second. Power of Accommodation : The ability of eye lens to adjust it focal length is called accommodation with the help of ciliary muscles. Ciliary Muslces Relaxed 1. Eye lens become thin 2. Increases the focal length 3. Enable us to see distant object clearly Near point of the Eye It is 25cm for normal eye. The minimum distance at which object can be seen most distinctly without strain. 115

Contract 1. Eye lens become thick 2. Decreases the focal length 3. Enable us to see nearby object clearly For point of the Eye It is infinity for normal eye. It is the farthest point upto which the eye can see object clearly.


DEFECTS OF VISION AND THEIR CORRECTION 1. CATARACT : The image can not be seen distinctly because eye lens become milky and cloudy. This condition is known as cataract, it can cause complete or partial loss of vision. This can be corrected by surgical removal of extra growth (cataract surgery) 2. Myopia : (Near Sightedness) A person can see nearby object clearly, but cannot see distant object distinctly. Image formed in front of the retina.

Normal Eye



Image formed at Retina



Myopic Eye

The Reason of defect 1. 2. Excessive curvature of eye lens (thick, decrease focal length) Elongation of the eye ball.

CORRECTION Corrected by using a Concave Lens of appropriate power.



Correction of Myopita



(3) Hypermetropia (Far - Sightedness) – A person cannot see nearby object clearly, but can see distant object distinctly. Image formed...
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