Page 157 - Class 10th Physics Human Eyes Note

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Human Eyes

1. The Human Eye : 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) is called Accomodation power .

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.

5. Hypermetropia (Long sightedness) : It is a kind of defect in human eye due to which a 

person can see distant objects properly but cannot see the nearby objects clearly. It happens due 

to 

(i) decrease in power of eye lens i.e., increase in focal length of eye lens.

(ii) shortening of eye ball.

6. Presbyopia : It is a kind of defect in human eye which occurs due to ageing. It happens due 

to

(i) decrease in flexibility of eye lens.

(ii) gradual weakening of cilliary muscles.

7. Astigmatism : It is a kind of defect in human eye due to which a person cannot see 

(focus) simultaneously horizontal and vertical lines both.

8. Cataract : Due to the membrane growth over eye lens, the eye lens becomes hazy or 

even opaque. This leads to decrease or loss of vision.

The problem is called cataract. It can be corrected only by surgery.

9. Dispersion of white light by a glass prism : The phenomenon of splitting of white light 

into its seven constituent colours when it passes through a glass prism is called dispersion 

of white light. The various colours seen are Violet, Indigo, Blue, Green, Yellow, Orange and 

Red. The sequence of colours remember as•VIBGYOR. The

band of seven colours is called spectrum.

10. Composition of white light : White light consists of seven colours i.e., violet, indigo, blue, 

green, yellow, orange and red.

11. Monochromatic light: Light consisting of single colour or wavelength is called 

monochromatic light, e.g., sodium light

12. Polychromatic light : Light consisting of more than two colours or wavelengths is 

called polychromatic light, e.g. white light.

13. Recombination of white light : Newton found that when an inverted prism be placed in 

the path of dispersed light then after passing through prism, they recombine to form white light.

14. Formation of rainbow : The water droplets act like small prisms. They refract and 

disperse the incident sunlight, then reflect it internally, and finally refract it again when it 

comes out of the raindrop. Due to the dispersion of light and internal reflection, different 

colours reach the observer’s eye.

15. Atmospheric Refraction : The refraction of light caused by the earth’s atmosphere (having 

air layers of varying optical densities) is called atmospheric refraction.

16. Colour of the Sun at sunrise and sunset : At noon, the light of sun travels relatively 

shorter distance through earth’s atmosphere thus appears white as only a little of blue and 

violet colours are scattered. Near the horizon, most of the blue light and shorter wavelengths 

are scattered and sun appears red.

17. Scattering of light:- When white light falls on certain types of substances, the particles of the substance absorb the light and then emit it again in all directions, this process is called scattering of light .

• Describe graphically the structure and function of the human eye.

Structure :-

The human eye is approximately spherical. The uppermost protruding membrane in front of it is called cornea. The colored layer behind it is called the iris. There is a small round hole in the middle of it. Which is called pupil. There is a convex lens behind it. Which is called eye lens. It remains suspended by the ciliary muscles through the suspensor ligament. The rearmost part of the eye is called the retina. It is connected to the brain through visual nerves.

Function:-

When rays of light travel from an object and fall on the eyes. So the real and inverted image of the eye lens is formed on the retina. Some substances present in the retina convert it into electrical signals. Which reaches the brain through the visual nerves and the brain again converts it into light energy and it perceives the direct image of the object.

Note :-

(i) Iris control the amount of light.

  (ii) Ciliary muscle controls the focal length of the eye lens.

• Defects of Eyes :-

The process of the eye not forming an image of an object on the retina is called Defects of the Eyes.

• Least distance of distinct vision :-

The minimum distance up to which objects can be seen clearly with normal eyes is called least distance of distinct vision.

It is denoted by D.

The minimum distance of clear vision is 25 CM.

• Normal. Eye :- The eye which does not have any defect is called normal eye.

• There are mainly three types of eye defects

(i) Hyper metropia or Long sightedness

(ii) Myopia or short sightedness

(i) Long sightedness:- If a person can see distant objects clearly. but cannot see the objects nearby clearly , the objects situated at a distance of 25 cm, then this defect of his eye is called long sightedness.

In this condition the image is formed behind the retina.

  To correct this defect, a convex lens is used.

(ii) Short sightedness:- If a person can see nearby objects clearly

But cannot see distant objects clearly, then this defect of his eye is called short sightedness.

In this stage the image is formed in front of the retina.

To remove this defect, a concave lens was used. goes.

• Far point:- The farthest point up to which a person with short-sightedness can see is called far point.

It is denoted by F.

  • Near Point: - The nearest point up to which a person with long sightedness can see. It is called near point.

  It is denoted by N

• Why does the sky appear blue from Earth?

  Ans:-

  When the rays of white light enter the atmosphere, the molecules and other particles of the atmosphere greatly distort the color present in the white light. In this way the blue color spreads all around. That is why the sky appears blue from Earth.

• Why does the sky appear black to an astronaut?

Ans:-

  An astronaut is in space at a place where there is no atmosphere. Therefore scattering does not occur there. That's why the sky appears black to them

• Why do the stars twinkle?

Ans:-

  Due to the changing density of the atmospheric layers, the rays of light coming from the star get refracted from these layers  and deviate sometimes less and sometimes more from their original path, due to which sometimes less or more light reaches the eyes, due to which the stars appear to twinkle.

• Moon and planets do not twinkle, why?

Ans:-

  The moon and planets are much closer to the Earth than the stars. Therefore they are considered to be an extensive source of light. Therefore, the visual angle made by the Moon and the planets is larger than the visual angle made by the stars. This is the reason why the moon and planets do not twinkle.

• Why does a diamond shine?

Ans:-

  The refractive index of diamond is 2.42. Due to high refractive index its critical angle is very small. Therefore, after cutting it properly, the light entering it is fully reflected from different places inside it due to which the diamond shines.

• Why does the black board appear black?

Ans:-

  The black board absorbs the white light falling on it. Thus it does not reflect any color. That is why the black board appears black.

Dear Asif Sir