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human eye and colourful world

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CBSE Gujarat Board Haryana Board

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Class 10 Class 12

What is meant by scattering of light? Use this phenomenon to explain why the clear sky appears blue or the Sun appears reddish at sunrise?


Scattering of light is the phenomenon in which a part of the incident light is dispersed in different directions.

Appearance of blue sky: The molecules of air and other fine particles in the atmosphere have a size smaller than the wavelength of visible light. So, they are more effective in scattering light of shorter wavelengths at the blue end than
light of longer wavelengths at the red end. Thus, when sunlight passes through the atmosphere, the fine particles in the air scatter blue colour (shorter wavelengths) more strongly than red. The scattered blue light enters our eyes, and hence, the clear sky appears blue.

Sun appears reddish at sunrise: At the time of sunrise and sunset, when the Sun is near the horizon, sunlight travels a greater distance through the atmosphere to reach us. During this, most of the shorter wavelengths present in it are scattered away from our line of sight by the molecules of air and other fine particles in the atmosphere. So, light reaching us directly from the rising or setting Sun consists mainly of the longer wavelength red colour because of which the Sun appears red. Thus, at sunrise and sunset, the Sun as well as the surrounding sky appears red.
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(a) List the parts of the human eye that control the amount of light entering into it.

Explain how they perform this function.

(b) Write the function of retina in human eye.

(c) Do you know that the corneal-impairment can be cured by replacing the defective cornea with the cornea of the donated eye? How and why should we organise groups to motivate the community members to donate their eyes after death?


a) The amount of light entering the eyes of a human is controlled by iris and pupil.

The iris automatically adjusts the size of the pupil according to the intensity of light received by the eye.

If the amount of light received by the eye is large (as during the daytime), then the iris contracts the pupil (makes the pupil small) and reduces the amount of light entering the eye. On the other hand, if the amount of light received by the eye is small (as in a dark room or during night), the iris expands the pupil (makes the pupil large) so that more light may enter the eyes. Thus, the iris regulates (or controls) the amount of light entering the eye by changing the size of the pupil.

b) The retina acts as a screen on which the image is formed in the eye.

The retina has a large number of light-sensitive cells called rods and cons. When the image falls on the retina then these light-sensitive cells get activated and generate electrical signals.

c) Yes.

There is a need for awareness among various communities about eye donation after death as it will help a person suffering from an eye ailment to get the treatment.

The following steps can be taken to organize groups to motivate them:

(i) The person who is taking the initiative can communicate with people and after realizing people who are like minded should gather them and form an awareness group.

(ii) The group should then approach various communities and organizations in their area and promote their campaign.

(iii) They can also approach various hospitals and clinics to promote the campaign.

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Write the importance of ciliary muscles in the human eye. Name the defect of vision that arises due to gradual weakening of the ciliary muscles. What types of lenses are required by the person suffering from this defect to see the objects clearly?

Akshay, sitting in the last row in his class, could not see clearly the words write on the blackboard. When the teacher noticed it, he announced if any student sitting in the front row could volunteer to exchange his seat with Akshay. Salman immediately agreed to exchange his seat with Akshay. He could now see the words written on the blackboard clearly. The teacher thought it fit to send the message to Akshay’s parents advising them to get his eyesight checked.

In the context of the above event, answer the following questions:

(a) Which defect of vision is Akshay suffering from? Which type of lens is used to correct this defect?

(b) State the values displayed by the teacher and Salman.
(c) In your opinion, in what way can Akshay express his gratitude towards the teacher and Salman?


Ciliary muscles present in the human eye enables us to adjust the curvature of the eye lens. This, in turn, changes the focal length of the eye lens.

Presbyopia is the defect in eye which arises because of the gradual weakening of ciliary muscles.
Bifocal lens is used to correct presbyopia.

a) Akshay is not able to see from a far distance, so he is suffering from myopia or nearsightedness.

Myopia can be corrected using a concave lens.

b) The teacher displayed presence of mind and pro-activeness, and she is of considerate nature.
Salman displayed a caring nature, kindness and taught the value of friendship.

c) Akshay can express his gratitude towards the teacher and Salman by thanking them in front of the whole class.

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The path of a ray of light passing through a glass prism is shown below:
 

In this diagram, the angle of prism, angle of incidence, angle of emergence and angle of deviation respectively have been represented by:

  • O, Y, Z and N

  •  P, Y, M and Z

  • O, X, M and Z

  • P, X, Z and N


C.

O, X, M and Z

The correct option is C.

O is the angle of prism.

Angle of incidence is the angle made by the incident ray with the normal to the surface of the prism. Here, X represents the angle of incidence.

Angle of emergence is the angle made by the emerging ray with the normal to the surface of the prism. Here, M represents the angle of emergence.

Angle of deviation is the angle made by the emerging ray with the incident ray. Here, Z represents the angle of deviation.

The correct option is C.

O is the angle of prism.

Angle of incidence is the angle made by the incident ray with the normal to the surface of the prism. Here, X represents the angle of incidence.

Angle of emergence is the angle made by the emerging ray with the normal to the surface of the prism. Here, M represents the angle of emergence.

Angle of deviation is the angle made by the emerging ray with the incident ray. Here, Z represents the angle of deviation.

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What is atmospheric refraction? Use this phenomenon to explain the following natural events:
a) Twinkling of stars
b) Advanced sunrise and delayed sunset
Draw diagrams to illustrate your answer.


Atmospheric refraction is the refraction of light caused by the earth's atmosphere due to change in the refractive indices of different layers of the atmosphere.

a) Twinkling of stars: Stars are distant point sourced sources of light. The path of the rays of light comig f=coming from the distant star goes on varying due to atmospheric refraction slightly. Thus, apparent position of the star fluctuates and the amount of star light entering the eye flickers giving the twinkling effect.


b) Advanced sunrise: When the sun is slightly below the horizon, light rays coming from the sun travel from rarer to denser layers of air. As a result of the atmospheric refraction of light. light appears to come from a higher position above the horizon. Therefore, the sun appears earlier than actual sunrise.

Delayed sunset: During sunset, the apparent position of the sun is visible to us and not the actual position due to the same bending of light rays effect. Therefore, because of refraction, we see the sunset afterwards although it has actually moved from its position.

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A student is observing a diagram showing the path of a ray of light passing through a glass prism. He would find that for all angles of incidence the ray of light bends:

  • Towards the normal while entering the prism and away from the normal while emerging from the prism.

  • Away from the normal while entering the prism and towards the normal while emerging from the prism.

  • Away from the normal while entering as well emerging from the prism.

  • Towards the normal while entering as well as while emerging from the prism.


A.

Towards the normal while entering the prism and away from the normal while emerging from the prism.

The correct option is A.

The ray of light travels from rarer to a denser medium as it enters a glass prism. Hence, ray of light bends towards the normal.

On emerging from the glass prism, ray of light travels from a denser medium to a rarer medium. Hence, ray of light bends away from the normal

The correct option is A.

The ray of light travels from rarer to a denser medium as it enters a glass prism. Hence, ray of light bends towards the normal.

On emerging from the glass prism, ray of light travels from a denser medium to a rarer medium. Hence, ray of light bends away from the normal
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