2.Human Eye and Colorful World-Full

Q.1 Describe the construction and working of a human eye.

Ans. Human eye:-The human eye is one of the most valuable and sensitive sense organs. It uses light and helps us to see the wonderful and colorful world. The eye is almost spherical in shape having a diameter of about 2.3cm.

Construction:- The main parts of a human eye are as:

 

1. Cornea:- The front of the eye is called cornea. It is made up of a transparent substance and is bulging outwards. It maintains the shape and size of the eye and protects its delicate inner structure. The light coming from the objects enters the eye through the cornea

2. Iris and Pupil:– Behind the cornea is a colored part called iris. It is a colored diaphragm which has a central circular opening called pupil. The pupil appears black because no light is reflectedfrom it. Pupil expands in dim light and contracts in bright day light.

3. Eye lens:- The eye lens is a convex lens made up of a transparent soft and flexible material like a jelly made of proteins. It forms a real and diminished image of an object on the screen (called retina).

4. Retina:- The screen on which the image is formed in the eye is called retina. It is located behind the eye lens and at the back part of the eye.

5. Ciliary Muscles:- The eye lens is held in the position by Ciliary muscles. By the action of Ciliary muscles, the focal length of the eye can be changed.

 

Working of Eye:- The light rays coming from the object kept in front of us enter the pupil of the eye and falls on the eye lens. The eye lens converges the light rays and produces a real and inverted image of the object on the retina. The image formed on the retina is conveyed to the brain by the optic nerve and thus this gives rise to the sensation of vision. Actually, the retina contains a large number of light sensitive cells which gets activated upon illumination and generate electric signals. These signals are then sent to brain via optic nerve. The brain interprets these signals and finally processes the information. The image on the retina is inverted, but our mind interprets the image as that of an erect image.

 

  

Q.2 Describe the function of Iris and pupil.

 Ans. The iris automatically adjusts the size of the pupil according the intensity of light received by the eye. If the amount of light received by the eye is high, then the pupil contracts and reduces the amount of light entering the eye. On the other hand, if the amount of light received by the eye is small, then the pupil expands to allow more light to enter the eye. Hence, the amount of light entering the eye can be controlled by the pupil of the eye.

 

Q.3 How do we see colors?

Ans. The retina of our eye has large number of light sensitive cells. These cells are of two types: rod shaped and cone shaped. The rod shaped cells respond to the intensity of light (or brightness of light) while the cone shaped cells respond to colors. So it is the cone shaped cells which makes us to distinguish between various colors.

 

Q.4 What is color blindness? What kind of retinal cells are lacking in a person suffering from this defect?

Ans. It is that defect of the eye due to which a person is unable to distinguish between certain colors. The color blinded person does not possess some cone shaped cells that responds to certain colors. It is genetic disorder and has not been cured so for.

 

Q.5 Why does it take some time to see objects in a dim room when we enter the room from bright sunlight?

Ans. The amount of light entering our eye is controlled by the pupil. When the intensity of light is more, pupil contracts and when the intensity is less, it expands. When we enter to a dim room from bright sunlight outside, less amount of light enters our due to narrow opening of pupil. Thus, we cannot see the object inside the dim room clearly at once, but gradually the pupil expands more and things start appearing normal.

 

Q.6 What do you mean by power of accommodation of eye?

Ans. A normal eye can focus the images of the distant as well as the nearby objects on its retina by changing the focal length of its lens. This ability of an eye to focus the distant as well as the nearby objects on the retina by changing the focal length of its lens is called accommodation.

   When the is looking at the distant object, the eye is said to be unaccommodated because it is the relaxed state of the eye while as when it is looking at a nearby object, the eye is said to be accommodated.

     A normal human eye can see the objects clearly which are lying anywhere between infinity to about 25cm. the power of accommodation of a normal human eye is about 4-dioptres.

 

Q.7 What is the far-point and near point of the eye?

Ans. Far Point:- The most distant point at which an object can be seen clearly is called the far point of the eye. E.g. For a normal eye, the far point lies at infinity

Near Point:- The point at closest distance at which an object can be seen clearly by the eye is called near point of the eye. E.g. For a normal eye, the near point is at a distance of 25cm.

 

Q.8 What do you mean by defects of vision? How are these defects corrected?

Ans. Defects of Vision:- In a normal eye. The image of an object is formed exactly on the retina. The eye is said to be defective, when the image of an object (distant or nearby object) is not formed on the retina.

   There are three common defects of vision as:

1. Myopia or Short sightedness:- Myopia is that defect of vision due to which a person cannot see the far-off objects clearly but can see only the nearby objects. E.g. A child suffering from this defect is unable to read the writings on the blackboard from the last bench in the classroom, although he/she can read his/her book comfortably.

  Myopia can be caused due to two reasons as:

I. either the converging power of lens is very high

II. or the eye ball may be larger than the normal size.

       This type of defect can be corrected by using spectacles fitted with concave lens with suitable power.

 The image of the object of a myopic eye is always formed in front of the retina.

        2. Hypermetropia or Long-Sightedness:- Hypermetropia is that defect of vision due to which a person cannot see the nearby objects clearly but can see the distant object only. E.g. A person suffering from this defect can not read a book or a newspaper clearly and comfortably but can read the writings placed at distant places.

  Hypermetropia can be also caused due to two reasons:

I. either the converging power of lens is very low

II. or the eye ball may be shorter than the normal size.

     The image of the object in a hypermetropic eye is always formed behind the retina.

This type of defect can be corrected by using spectacles containing convex lens of suitable power.

3. Presbyopia:- It is that defect of vision due to which an old aged person cannot see the nearby objects clearly due to the loss of the power of accommodation of the eye. It is a special kind of the Hypermetropia. Presbyopia occurs in old age due to gradual weakening of the Ciliary muscles and diminishing flexibility of the eye lens.

   This type of defect is corrected in the same way as Hypermetropia by using spectacles having convex lenses.

 

Q.9 Distinguish between human eye and camera.

Ans.

    

Human Eye	Camera
1. The focal length of the eye can be changed by the action of Ciliary muscles.   2. The image formed on the retina of the eye is not permanent.   3 Retina can be used again and again for forming the image.   4. The focusing is done by changing the focal length of the eye lens.	1. The focal length of a camera is fixed and thus it cannot be changed.   2. The photographic film a camera retains the image of the object permanently.   3. A photographic film can be used only once for forming the image.   4. The focusing is done by changing the distance between lens and the film.

 

Q.10 What do you mean by dispersion of light?

Ans. Dispersion of Light:- It was Newton who discovered in 1665 by his experiment with glass prism that white light is a mixture of seven colors. He found that if a beam o light is passes through a glass prism, it splits to form a band of seven colors on a white screen. The phenomenon of splitting of white light into seven colors is called dispersion of light and the band of seven colors obtained on a screen is spectrum of white light. The seven colors of the spectrum can be denoted by the word VIBGYOR, where V stands for violet, I for indigo, B for blue, Y for yellow, O for orange and R for red.

   The dispersion of white light occurs because the angle of refraction of different colors is different when passing through the glass prism.

 

   

 

 

Q.11 What is rainbow? How is it formed?

Ans. Rainbow:- The rainbow is an arch of seven colors visible in the sky which is produced by the dispersion of sun’s light by raindrops in the atmosphere.

     Just after rain or during rain, there is a large number of raindrops in the atmosphere which acts like small prisms. As white light enters and leaves these water drops, the various colored rays present in white light are refracted by different amounts due to which an arch of seven colors called rainbow is formed in the sky.

 

 Q.12 What do you mean by atmospheric refraction?

Ans. Atmospheric Refraction:- We know that, in the atmosphere the temperature of air is not same everywhere, but some of the air layers are cold and some are comparatively warm. The cooler layers of the atmosphere behave as optically denser medium for light rays while the warm air layers behave as optically rarer medium. When light rays passes through these air layers of different optical densities, then refraction (or bending) of light takes place. Thus the refraction of light caused by the earth’s atmosphere (having air layers of different optical densities) is called the atmospheric refraction.

 

 Q.13 What do you mean by twinkling of stars? What is the cause of twinkling of stars?

Ans. Twinkling of Stars:- When we look at a star in the sky on a clear night, we observe that the intensity of light coming from it continuously changes. At one moment the star appears bright and in the next moment the star appears dim. In fact, the star go on becoming bright and dim again and again. This brightening and diming of a star again and again is called twinkling of a star.

 

Cause of twinkling of Stars:- The twinkling of a star is due to the atmospheric refraction of star light. When the light coming from a star enters the atmosphere, it undergoes refraction due to different optical densities of air at different levels. The continuously changing atmosphere refracts the from the star by different amounts from one moment to the next. Sometimes, the atmosphere refracts more star light towards us, then the star appears to be bright and sometimes it refracts less star light and thus the star appears to be dim in this way, star light reaching our eyes increases and decreases continuously due to atmospheric refraction and thus the star appears to twinkle.

 

Q.14 Why do stars seemhigher than they actually are?

Ans. We know that the air layer near the ground level is optically denser and becomes rear and rear as we go higher and higher above the earth. When light from a star passes through various air layers of different densities, it bend more towards the normal near the ground and thus due to atmospheric refraction, the star ‘S’ appears in the position ‘s*’ at a height greater than its actual height.

 

                     

 

Q.15 The rays of sun remains visible to us for sometimes after the sunset (or before sunrise). Why?

Ans. As light rays from the sun passes through different atmospheric layers, they bend towards the normal on passing from rarer to denser layers with the result the apparent position of sun seems to be at S* instead of S. Thus for about 2-minutes even after setting of sun, the apparent position appears above the horizon and an observer can see it.

   Similarly, the sun becomes visible in the morning nearly 2-minutes before rising above the horizon.

 

 

Q.16 What is Tyndall effect?

Ans Tyndall Effect:- Scattering of light is the phenomenon of change in the direction of the light on striking the various suspended particles in its path. The scattering of light by particles in its path is called Tyndall effect.

 E.g. When a beam of light enters a room through a window, the dust particles suspended in the air becomes visible due to the scattering of light by these dust particles.

 

Q.17 Why the sun appears red at sunset and sunrise?

Ans. At the time of sunrise and sunset, the sunlight has to travel the greatest distance through the atmosphere to reach us. During this long journey of sun light, most of the shorter wavelength blue color present in it, is scattered out. So, the light reaching us directly from the rising sun or setting sun consists mainly of larger wavelength, red color due to which the sun appears red.

 

Q.19 Why does sky appears blue?

Ans. The molecules of air (like oxygen, nitrogen etc) in the atmosphere have size smaller than t he wavelength of the visible light. These are more effective in scattering light of shorter wave lengths at the blue end. Thus, when sunlight passes through the atmosphere, the particles in air scatter blue color more strongly than the red color. The scattered blue light enters our eyes and thus, it appears that the sky is blue in color