Human Eye and Colourful World Class 10 Important Questions with Answers Science Chapter 11

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Class 10 Science Chapter 11 Important Questions with Answers Human Eye and Colourful World

Class 10 Chemistry Chapter 11 Important Questions with Answers Human Eye and Colourful World

Human Eye and Colourful World Class 10 Important Questions Very Short Answer Type

Question 1.
What is Tyndall effect? (2011 D)
Answer:
The scattering of a beam of light is called the Tyndall effect. Tyndall effect can be observed when sunlight passes through the canopy of a dense forest. In a dense forest, the mist contains tiny droplets of water, which act as particles of colloid dispersed in air.

Question 2.
Give an example of a phenomenon where Tyndall effect can be observed. (2011 OD)
Answer:
The scattering of light by particles of colloid or suspension in its path is called Tyndall effect. When sunlight passes through the canopy of a dense forest, the tiny water droplets in the mist scatter sunlight.

Question 3.
What is the colour of scattered sunlight when the size of the scattering particles is relatively large? (2011 OD)
Answer:
When white sunlight falls on relatively larger particles (like dust particles, etc.) present in the atmosphere, it is scattered as such so the scattered light appears blue.

Question 4.
What will be the colour of the sky when it is observed from a place in the absence of any atmosphere? (2012 D)
Answer:
The colour of sky will be black when it is observed from a place where atmosphere is absent as scattering of light does not take place when there is no atmosphere.

Question 5.
The sky appears dark instead of blue to an astronaut. State its reason. (2012 D)
Answer:
There is no atmosphere containing air in space to scatter sunlight. As there is no scattering of light in space, the scattered light does not reach the eyes and the sky appears dark instead of blue to an astronaut in outer space.

Question 6.
Which phenomenon is responsible for making the path of light visible? (2012 D)
Answer:
Tyndall effect is responsible for making the path of light visible.

Question 7.
State one function of iris in human eye. (2012 D)
Answer:
Function of iris: Iris regulates the amount of light entering the eye by adjusting the size of the pupil.

Question 8.
State one function of pupil in human eye. (2012 OD)
Answer:
The amount of light entering the eye can be controlled by the pupil. If the intensity of outside light is low, then the pupil expands to allow more light to enter the eye. On the other hand, if outside intensity of light is high, then the pupil contracts so that less light enters the eye.

Question 9.
State one role of ciliary muscles in the human eye. (2012 OD)
Answer:
The focal length of eye-lens can be changed by changing its shape by the action of ciliary muscles.

Human Eye and Colourful World Class 10 Important Questions Short Answer Type I

Question 1.
Name the type of particles which act as prisms in the formation of rainbow in the sky. (2011 OD)
Answer:
The rain drops in the atmosphere act like many small prisms. As white sunlight enters and leaves these raincirops, the white light splits into an arch of seven colours of rainbow.

Question 2.
Explain with the help of a diagram, how we are able to observe the sunrise about two minutes before the Sun gets above the horizon. (2011 OD)
Answer:
The Sun can be seen about two minutes before sunrise because when the Sun is slightly below the horizon, the Sun’s light coming from less dense air to more dense air is refracted downwards as it passes through the atmosphere. Thus due to this atmospheric refraction, the Sun appears to be raised above the horizon when actually it is slightly below the horizon.

Human Eye and Colourful World Class 10 Important Questions Short Answer Type II

Question 1.
(a) What is meant by the dispersion of white light? Draw a diagram to show dispersion of white light by the glass prism.
(b) Explain why the planets do not twinkle but the stars twinkle.
Answer:
(a) Dispersion of light. The splitting of white light into its component colours on passing through a prism is called dispersion of light.

(b) The planets are much closer to the earth. A planet can be considered as a collection of large number of point-sized sources of light. So the total variation in the amount of light entering our eye from all the individual point-sized sources will average out to zero thereby nullifying the twinkling effect.

On the other hand, stars twinkle because stars are point-sized sources of energy therefore the continuously changing atmosphere causes atmospheric refraction which causes variation in light.

Question 2.
(a) What is meant by the power of accommodation of an eye?
(b) A person with a myopic eye cannot see objects beyond 1.2 m directly. What should be the type of the corrective lens used? What would be its power? (2011 OD)
Answer:
(a) The ability of an eye to focus the distant objects as well as the nearby objects on the retina by changing the focal length or converging power of its lens is called power of accommodation.
A normal eye has a power of accommodation which enables objects as far as infinity and as close as
25 cm to be focussed on the retina.

(b) Myopic eye
v = 1.2 m = -120 cm;
u = (infinity);
P of the corrective lens = ?
By Lens formula,

Thus the corrective lens for the above defect is concave lens (as focal length has -ve sign) of focal length of 120 cm.

Question 3.
What eye defect is hypermetropia? Describe with a ray diagram how this defect of vision can be corrected by using an appropriate lens.  (2011 OD)
Answer:
Long sightedness is hypermetropia. Due to this defect, a person is not able to see the nearby objects clearly but can see the distant objects clearly.

Causes of long-sightedness:
It is caused due to the following reasons:

• Normal increase in the focal length of the eye lens. The lens becomes less convergent.
• Shortening of the eyeball size.

Long sightedness can be corrected by using a convex lens of suitable focal length in the spectacles of such a person.

When a convex lens of suitable power is placed in front of the hypermetropic eye then the diverging rays of light coming from the nearby object are first converged by this convex lens. Due to this, the convex lens forms a virtual image of the nearby object at a point near to the hypermetropic eye. Then the hypermetropic eye can easily focus the image formed by convex lens on the retina.

An eye suffering from long-sightedness, far sightedness or hypermetropia. The image I of a nearby object 0 is formed behind the retina

Long-sightedness is corrected by using a convex lens

Question 4.
What eye defect is myopia? Describe with a neat diagram how this defect of vision can be corrected by using a suitable lens. (2011 OD)
Answer:
Myopia is the defect of the eye vision due to which a person can see the near objects clearly, but he cannot see the far objects clearly.

Causes of myopia. Myopia is caused

• Due to the elongation of the eye ball.
• Due to decrease in the focal length of the eye lens. The eye lens becomes more convergent.

Myopia can be corrected by using a concave lens of suitable focal length in the spectacles of such a person.
Ray diagram to illustrate this defect

Ray diagram to show the correction of myopia.

Question 5.
What is a spectrum? Why do different coloured rays deviate differently on passing through a glass prism? (2011 OD)
Answer:
Meaning of spectrum. When a beam of white light is passed through a prism, a band of seven colours is formed on a white screen. This is called spectrum of white light.

The dispersion of white light occurs because the angle of refraction or angle of bending of different components of the white light is different when passing through the glass prism. When white light consisting of seven colours falls on a prism, each colour is refracted or deviated by a different angle, with the result that the seven coloured band, i.e., spectrum is formed.

Question 6.
A student cannot see a chart hanging on a wall placed at a distance of 3 m from him. Name the defect of vision he is suffering from. How can it be corrected? Draw ray diagrams for the (i) defect of vision and also (ii) for its correction. (2012 D)
Answer:
If a student cannot see the object clearly placed at a distance of 3 m from him then he is suffering from Myopia or Short Sightedness.
Ray diagrams for the (i) defect of vision and (ii) for its correction:

Question 7.
(a) Draw a labelled ray diagram to illustrate the dispersion of a narrow beam of white light when it passes through a glass prism.
(b) A star appears slightly higher (above) than its actual position in the sky. Illustrate it with the help of a labelled diagram. (2012 OD)
Answer:
(a)

(b) Light from a star is refracted as it leaves vacuum and enters the earth’s atmosphere. Air at higher altitudes is rarer and is denser nearer to the earth’s surface. Thus when light coming from the star comes down, the dense air bends the light more. Due to this refraction of star’s light, the star appears to be at a higher position.

Question 8.
What is the colour of the clear sky during day-time? Give reason for it. (2012 OD)
Answer:
During day time the colour of the sky is blue.
Reason: The molecules of air and other fine particles in the atmosphere have a size smaller than the wavelength of visible light. So these particles scatter more effectively the light rays of shorter wavelength at the blue end than light of longer wavelengths at the red end. When the scattered blue light enters our eyes, it gives us the feeling of a blue sky.

Question 9.
State the difference in colours of the Sun observed during sunris^sunset and noon. Give explanation for each. (2013 D)
answer:
The Sun and surrounding sky appear red at sunrise and at sunset because at this time the Sun is near the horizon and sunlight has to travel the greatest distance through the atmosphere to reach us. Thus most of the blue colour present in sunlight has been scattered out and away from our line of sight, leaving behind mainly red colour in the direct sunlight beam that reaches our eyes.

When the Sun is overhead (as at noon) then the light coming from the Sun has to travel a relatively shorter distance through the atmosphere to reach us. Thus only a little of blue colour of the white light is scattered. Since the light coming from the overhead Sun has almost all its component colours in the right proportion, therefore the Sun in the sky overhead appears white.

Question 10.
When and where do we see a rainbow? How is a rainbow formed? Draw a labelled diagram to illustrate the formation of a rainbow. (2013 OD)
Answer:
Rainbow is caused by dispersion of sunlight by tiny water droplets present in the atmosphere. The water droplets act like small prisms. They refract the incident sunlight and then reflect it internally and finally refract it again when it comes out of the rain-drop. A rainbow is always formed in the direction opposite to that of the Sun.

Question 11.
Explain giving reason why the sky appears blue to an observer from the surface of the earth? What will the colour of the sky be for an astronaut staying in the international space station orbiting the earth? Justify your answer giving reason. (2014 D)
Answer:
Sky appears blue. The molecules of air and other fine particles in the atmosphere have a size smaller than the wavelength of visible light. So these particles scatter more effectively the light rays of shorter wavelength at the blue end than light of longer wavelengths at the red end. When the scattered blue light enters our eyes, it gives us the feeling of a blue sky.

Colour of sky to an astronaut. There is no atmosphere containing air in space to scatter sunlight. As there is no scattering of light in space, the scattered light does not reach the eyes and the sky appears dark instead of blue to an astronaut in outer space.

Question 12.
Explain with help of a labelled diagram, the cause of twinkling of stars. (2014 D)
Answer:
Twinkling of Stars. The twinkling of a star is due to atmospheric refraction of star light. The atmospheric refraction occurs in a medium of gradually changing refractive index. Since the atmosphere bends star light towards the normal, the apparent position of the star is slightly different from the actual position.

This apparent position of the star is not stationary, but keeps on changing slightly as the physical conditions of the earth’s atmosphere are not stationary.

Since the stars are very distant, they act as point-sized sources of light. As the path of rays of light coming from the star goes on varying slightly, the apparent position of the star fluctuates and the amount of light entering the eye flickers – the star sometimes appears brighter and at other times appears fainter, which is the twinkling effect.

Question 13.
A narrow beam PQ of white light is passing through a glass prism ABC as shown in the diagram.
(i) Write the name and cause of the phenomenon observed.
(ii) Where else in nature is this phenomenon observed?
(iii) Based on this observation, state the conclusion which can be drawn about the constituents of white light. (2014 OD)

Answer:
(i) The phenomenon of splitting of white light into seven colours on passing through a glass-prism is called dispersion of light. The dispersion of white light occurs because colours of white light travel at different speeds through the glass prism.
(ii) Formation of rainbow is an example of dispersion of white light in nature.
(iii) The formation of spectrum of seven colours shows that white light is made up of lights of seven different colours mixed together. That is, white light is a mixture of seven colours (or seven coloured lights), i.e., red, orange, yellow, green, blue, indigo and violet.

Question 14.
With the help of a labelled diagram, explain why the sun appears reddish at the sunrise and the sunset. (2015 D)
Answer:
At the time of sunrise and sunset when the sun is near the horizon, the sunlight has to travel the greatest distance through the atmosphere to reach us. During the long journey of sunlight, most of the shorter wavelength blue-colour present in it is scattered out and away from our line of sight so, the light reaching us directly from the rising sun or setting sun consists mainly of longer wavelength red colour due to which the sun appears red. Due to the same reason, the sky surrounding the rising sun and setting sun also appears red. Thus, at sunrise and sunrise, the sun itself, as well as the surrounding sky appear red.

Question 15.
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. (2015 OD, 2016 D)
Answer:
Scattering of light is the phenomenon to throw light in various random directions. Light is scattered when it falls on various types of suspended particles in its path. The colour of the scattered light depends on the size of the scattering particles in the atmosphere.

• The larger particles of dust and water droplets present in the atmosphere scatter the light of longer wavelengths due to which the scattered light appears white.
• The extremely minute particles such as air molecules present in the atmosphere scatter mainly blue light present in the white sunlight.

Colour of the sky appears blue. The molecules of air and other fine particles in the atmosphere have a size smaller than the wavelength of visible light. So these particles scatter more effectively the light rays of shorter wavelength at the blue end than light of longer wavelength at the red end. When the scattered blue light enters our eyes, it gives us the feeling of a blue sky.

Colour of the sun appears red at sunrise and sunset. The sun at sunrise and sunset is very near to the horizon, and near the horizon most of the blue light of shorter wave-lengths is scattered away by the particles in the atmosphere. Therefore, the light that reaches our eyes is of longer wavelengths that gives rise to the reddish appearance of the sun.

Question 16.
Why does the sun appear reddish early in the morning? Will this phenomenon be observed by an observer on the moon? Justify your answer with a reason.  (2016 D)
Answer:
The sun at sunrise (early in the morning) is located near the horizon of the earth. Light from the sun near the horizon has to pass through thick layers of air and a large distance through the earth’s atmosphere before reaching our eyes. Near the horizon, most of the blue light rays with shorter wavelength are scattered away by the particles in the atmosphere. Therefore, the light that reaches our eyes is the red light of longer wavelengths. This gives rise to the reddish appearance of the sun.

This phenomenon will not be observed by an observer on the moon because there is no atmosphere on the moon to scatter light.

Question 17.
State the cause of dispersion of white light passing through a glass prism. How did Newton show that white light of Sun contains seven colours using two identical glass prisms. Draw a ray diagram to show the path of light when two identical glass prisms are arranged together in inverted position with respect to each other and a narrow beam of white light is allowed to fall obliquely on one of the focus of the first prism. (2016 D)
Answer:
White light is a mixture of lights of seven colours, red, orange, yellow, green, blue, indigo and violet. The dispersion of white light occurs because colours of white light travel at different speeds through the glass prism. The amount of refraction depends on the speed of coloured light in glass.

When white light consisting of seven colours falls on a glass prism, each colour in it is refracted by a different angle, with the result that seven colours are spread out to form a spectrum. The red light bends the least, while violet bends the most.

Newton’s experiment with two identical prisms:

• When a beam of white light is passed through a glass prism, a band of seven colours is formed on a white screen. This band of seven colours is called spectrum of white light.
• Newton showed that the seven coloured lights of the spectrum can be recombined to give back white light.
• First he tried to split the colours of the spectrum of white light using a prism.
• He then placed a second identical prism in an inverted position with respect to the first prism. This allowed all the colours of the spectrum to pass through the second prism. He found a beam of white light emerging from the other side of the second prism.

Question 18.
Due to gradual weakening of ciliary muscles and diminishing flexibility of the eye lens a certain defect of vision arises. Write the name of this defect. Name the type of lens required by such persons to improve the vision. Explain the structure and function of such a lens. (2017 D)
Answer:
The defect of this vision is called Presbyopia.
Bifocal lens is used for the correction of this defect. The upper part of this lens is concave and the lower part of the lens is Convex lens. The upper part of the lens is used to see the distant objects clearly and lower part of this lens is used to see the nearby objects clearly.

Human Eye and Colourful World Class 10 Important Questions Long Answer Type

Question 1.
Nalin and his four friends were sitting on his roof on a pleasant day. All of them were enjoying Ludo. Suddenly Ayush saw seven colours in the sky. He jumped with joy and shouted “Look, there is an Indradhanush in the sky”. Then Nalin explained all about the rainbow. After that every one clapped for him.
(a) What information is given by Nalin to his friends about a rainbow?
(b) Is it possible to obtain rainbow phenomenon on the earth?
(c) Which term is used for the seven colours of the rainbow?
(d) Which colour appears at the top and at the bottom of the rainbow? (2011 D)
Answer:
(a) Rainbow is a natural phenomenon in which an arch of seven colours visible in the sky is produced by the dispersion of white sunlight by raindrops in the atmosphere. Each raindrop acts as a tiny glass prism splitting the sunlight into seven colours.
(b) Yes, in daily life, when white light of the Sun is passed through a glass prism, it splits into seven colours.
(c) The band of seven colours obtained by the splitting of white light is called spectrum.
(d) The red colour appears at the top of the rainbow whereas violet colour appears at its bottom.

Question 2.
(a) What are the values of (i) near point and (ii) far point of vision of a normal adult person?
(b) A student has difficulty in reading the blackboard while sitting in the last row. What could be his defect of vision? Draw a ray diagram to illustrate this defect of vision. (2011 OD)
Answer:
(a) (i) The nearest point up to which the eye can see an object clearly without any strain is called the near point of the eye. The near point of a normal human eye is at a distance of 25 cm from the eye.

(ii) The farthest point from the eye at which an object can be seen clearly is known as the far point of the eye. The far point of a normal human eye is at infinity.

(b) This defect of vision is called myopia.
Ray diagram:
Ray diagram to illustrate this defect

Ray diagram to show the correction of myopia

Question 3.
Draw a diagram to show the dispersion of white light by a glass prism. What is the cause of this dispersion? (2011 OD)
Answer:
The splitting up of white light into seven colours on passing through a transparent medium like a glass prism is called dispersion of light.

We get different colours of light when white light is passed through a glass prism because white light is a mixture of lights of seven colours: red, orange, yellow, green, blue, indigo and violet. The dispersion of light occurs because the angle of refraction (or angle of bending) of lights of different colours is different when passing through the glass prism.

Question 4.
A person cannot read newspaper placed nearer than 50 cm from his eyes. Name the defect of vision he is suffering from. Draw a ray diagram to illustrate this defect. List its two possible causes. Draw a ray diagram to show how this defect may be corrected using a lens of appropriate focal length. (2013 D)
Answer:
If a person cannot read newspaper nearer than 50 cm from his eyes then he is suffering from hypermetropia. It is also called long-sightedness.
Ray diagram, causes and correction of this defect.

An eye suffering from long sightedness, far sight edness or hypermetropia. The image I of a nearby object O is formed behind the retina.

Long sightedness is corrected by using a convex lens

Question 5.
(a) A student cannot see clearly a chart hanging on a wall placed at a distance of 3 m from his eyes. Name the defect of vision he is suffering from. Draw a ray diagram to illustrate this defect. List its two possible causes.
(b) Draw a ray diagram to show how this defect may be corrected using a lens of appropriate focal
length. (2012 OD)
Answer:
Myopia is the defect of the eye vision due to which a person can see the near objects clearly, but he cannot see the far objects clearly.
Causes of myopia. Myopia is caused

• Due to the elongation of the eye ball.
• Due to decrease in the focal length of the eye lens. The eye lens becomes more convergent.
  Myopia can be corrected by using a concave lens of suitable focal length in the spectacles of such a person.

(b) Ray diagram to illustrate this defect

Ray diagram to show the correction of myopia.

Question 6.
List three common refractive defects of vision. Suggest the way of correcting these defects. (2014 D) Answer:
The three defects of vision:
(i) Myopia: Myopia is the defect of the eye vision due to which a person can see the near objects clearly, but he can not see the far objects clearly.

Causes of myopia: Myopia is caused due to the elongation of the eye ball and due to decrease in the focal length of the eye lens. The eye lens becomes more convergent.

Correction: Myopia can be corrected by using a concave lens of suitable focal length in the spectacles of such a person.

(ii) Long-sightedness or hypermetropia. Due to this defect, a person is not able to see the nearby objects clearly but can see the distant objects clearly.
Causes of hypermetropia: It is caused due to the following reasons:

• Normal increase in the focal length of the eye lens. The lens becomes less convergent.
• Shortening of the eyeball size.

Correction: Long sightedness can be corrected by using a convex lens of suitable focal length in the spectacles of such a person.

When a convex lens of suitable power is placed in front of the hypermetropic eye then the diverging rays of light coming from the nearby object are first converged by this convex lens. Due to this, the convex lens forms a virtual image of the nearby object at a point near to the hypermetropic eye. Then the hypermetropic eye can easily focus the image formed by convex lens on the retina.

(iii) Presbyopia. The power of accommodation of the eye decreases with ageing. It occurs due to the gradual weakening of the ciliary muscles and decreasing flexibility of the crystalline lens. The near point of the eye increases with age. It may reach even 2 metres. This defect is called presbyopia.

Causes of presbyopia: It is mainly caused due to the weakening of the ciliary muscles of the eyes. Such a person may suffer from myopia and hypermetropia.

Correction: This defect is then corrected by using bifocal lenses of suitable focal lengths. The upper part of the lens is a concave lens for correcting myopia to see the distant objects clearly, while the lower part of the lens has a convex lens to correct the hypermetropia to see the nearby objects clearly.

Question 7.
(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. (2014 OD)
Answer:
(a) The iris and pupil control the amount of light entering the eyes. If the amount of light received by the eye is large (as during the day time), then the iris contracts the pupil (makes the pupil small) and reduces the amount of light entering the eye.

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.

(b) The screen on which the image is formed in the eye is called retina. Retina is a delicate membrane having a large number of light sensitive cells called ‘rods’ and ‘cones’ which respond to the ‘intensity of light’ and ‘colours of object’ respectively.

Question 8.
A student is unable to see clearly the words written on the blackboard placed at a distance of approximately 4 m from him.
Name the defect of vision the boy is suffering from. Explain the method of correcting this defect. Draw ray diagram for the:
(i) defect of vision and also (ii) for its correction.  (2015 D)
Answer:

• As the student is unable to see clearly the words written on the blackboard at a distance of 4 m, the student is suffering from myopia or near-sightedness.
• This defect is corrected by using spectacles fitted with concave lens of suitable focal length is used. The image is allowed to form at the retina by using a concave lens of suitable power.

Ray diagrams for the defect and for its corrections:
Ray diagram to illustrate this defect

Ray diagram to show the correction of myopia

Question 9.
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 in old age. What type of lenses are required by the persons suffering from this defect to see the objects clearly?
Akshay, sitting in the last row in his class, could not see clearly the words written 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: (2015 OD)
Which defect of vision is Akshay suffering from? Which type of lens is used to correct this defect?
Answer:
Importance of ciliary muscles. The focal length of eye lens can be changed by changing its shape by the action of ciliary muscles. Ciliary muscles modify the curvature of the eye lens to enable the eye to focus objects at varying distances.

In old age, ciliary muscles become weak and the eye lens become inflexible. As a result, the eye loses its power of accommodation. Because of this, an old person cannot see the nearby objects clearly. This leads to the defect called presbyopia.

Bifocal lenses are required by the persons suffering from presbyopia in which upper part consists of a concave lens for distant vision and the lower part consists of a convex lens used to see nearby objects. Akshay is suffering from myopia or near sightedness. Myopia is corrected by using spectacles containing concave lenses. The concave lens used for correcting myopia should be of such a focal length that it produces a virtual image of the distant object (lying at infinity) at the far point of the myopic eye.

Question 10.
(a) Write the function of each of the following parts of human eye:
cornea; iris; crystalline lens; ciliary muscles
(b) Millions of people of the developing countries of the world are suffering from corneal blindness. These persons can be cured by replacing the defective cornea with the cornea of a donated eye. A charitable society of your city has organised a campaign in your neighbourhood in order to create awareness about this fact. If you are asked to participate in this mission how would you contribute in this noble cause?
(i) State the objective of organising such campaigns.
(ii) List two arguments which you would give to motivate the people to donate their eyes after death. (2016 D)
Answer:
(a) Functions of the following parts of human eye:
(i) Cornea. The front part of the eye is called cornea. It is made up of a transparent substance. The light coming from objects enters the eye through cornea.
(ii) Iris. This is a flat, coloured, ring-shaped membrane behind the cornea. Pupil is a hole in the middle of the iris. Iris controls the size of the pupil.
(iii) Crystalline lens. Eye lens is a convex lens which focuses the image of the object on the retina.
(iv) Ciliary muscles. Ciliary muscles hold the eye lens and changes the thickness of eye-lens while focussing the objects placed at different distances from the eye.

(b) (i) Objective of such campaigns. To make people aware of corneal blindness and make them
realise their duties towards the society by taking pledge for eye donation.
(ii)

• One pair of eyes can give eyesight to two corneal blind persons (each getting one eye), and make them see this beautiful world.
• Our eyes can live even after our death. People belonging to all age groups, even people with medical conditions like cataract, diabetes, hypertension can donate their eyes.

Question 11.
What is atmospheric refraction? Use this phenomenon to explain the following natural events.
(a) Twinkling of stars
(b) Advanced sun-rise and delayed sunset
Draw diagrams to illustrate your answers.  (2016 OD)
Answer:
Atmospheric refraction. The refraction of light caused by the earth’s atmosphere having air layers of varying optical densities is called atmospheric refraction.
(a) Twinkling of stars.
Twinkling of Stars. The twinkling of a star is due to atmospheric refraction of star light. The atmospheric refraction occurs in a medium of gradually changing refractive index. Since the atmosphere bends star light towards the normal, the apparent position of the star is slightly different from the actual position.

This apparent position of the star is not stationary, but keeps on changing slightly as the physical conditions of the earth’s atmosphere are not stationary.

Since the stars are very distant, they act as point-sized sources of light. As the path of rays of light coming from the star goes on varying slightly, the apparent position of the star fluctuates and the amount of light entering the eye flickers – the star sometimes appears brighter and at other times appears fainter, which is the twinkling effect.

(b) Advanced sun-rise and delayed sun-set. The Sun can be seen about two minutes before sunrise because when the Sun is slightly below the horizon, the Sun’s light coming from less’ dense air to more dense air is refracted downwards as it passes through the atmosphere. Thus due to this atmospheric refraction, the Sun appears to be raised above the horizon when actually it is slightly below the horizon.

It is also due to atmospheric refraction that we can still see the sun for about 2 minutes even after the Sun has set below the horizon. At sunset, the apparent position of the sun is visible to us and not the actual position because of the bending of light rays effect.
So, the time from sunrise to sunset is lengthened by about 2 + 2 = 4 minutes because of atmospheric refraction.

Question 12.
(a) Draw a ray diagram to explain the term angle of deviation.  (2017 D)
(b) Why do the component colours of incident white light split into a spectrum while passing through a glass prism, explain.
(c) Draw a labelled ray diagram to show the formation of a rainbow.
Answer:
(a) Angle of deviation is the angle between extended incident ray and extended emergent ray through a prism.
PQ = Incident Ray
RS = Emergent Ray
∠D = Angle of Deviation

(b) The incident white light splits into a seven coloured spectrum as speed of different coloured light waves is different when these pass through the prism.

(c)

Question 13.
(a) A student suffering from myopia is not able to see distinctly the objects placed beyond 5 m. List two possible reasons due to which this defect of vision may have arisen. With the help of ray diagrams, explain
(i) why the student is unable to see distinctly the objects placed beyond 5 m from his eyes.
(ii) the type of the corrective lens used to restore proper vision and how this defect is corrected by the use of this lens.
(b) If, in this case, the numerical value of the focal length of the corrective lens is 5 m, find the power of the lens as per the new Cartesian sign convention.  (2017 OD)
Answer:
(a) Two possible reasons:

• The eye ball size might have got elongated.
• The eye lens is more thick than its normal thickness during its power of accommodation.

(i)

(ii)

(b) f = -5 m (since lens is concave); P = ?
P = \(\frac{1}{f}=\frac{1}{-5}\) = -0.2 Diopter
Power of correcting lens is -0.2 D and this correcting lens is concave lens.