# CBSE Solutions for Class 10 English

#### GSEB std 10 science solution for Gujarati check Subject Chapters Wise::

Name the type of a lens used to correct hypermetropic eye.

જવાબ : Convex lens.

What is hypermetropia or far-sightedness ? [CBSE (Foreign) 2008]

જવાબ : A human eye which can see distant objects clearly but cannot see the near objects clearly is said to be suffered with a defect called hypermetropia or far sightedness.

Mention the value of range of vision for normal eye. (CBSE 2014, 2016)

જવાબ : Range of vision for normal eye is 25 cm to infinity.

Mention the value of far point for normal eye. (CBSE 2014, 2015)

જવાબ : Infinity.

Mention the value of near point for normal eye. (CBSE 2014, 2015)

જવાબ : 25 cm.

Name the part of human eye which acts as a screen to obtain the image of an object. (CBSE 2012, 2014)

જવાબ : Retina.

What is the nature of the image formed at the retina of human eye ?

જવાબ : Real and inverted image.

Why does the sky appear dark instead of blue to an astronaut?  [2011, 2012]

જવાબ : This is because there is no scattering of light due to the absence of atmosphere in the free space.

Name the ability of eye lens to adjust its focal length.

જવાબ : Accommodation.

Explain why the planets do not twinkle. [2011,2012,2015]

જવાબ : Planets are comparatively much closer to the earth and are thus seen as extended sources of light. Being of larger size, planets can be taken as a collection of a number of point-sized sources of light which nullify the twinkling effect of each other.

What happens to the image distance in the eye when we increase the distance of an object from the eye?   [2011]

જવાબ : In order to focus the eye on objects situated at varying distances, the focal length of the eye lens is changed by the action of ciliary muscles.

Why is a normal eye not able to see clearly the objects placed closer than 25 cm?  [2011]

જવાબ : A normal eye is not able to see clearly the objects placed closer than 25 cm because thefocal length of eyelens cannot be decreased below a certain minimum limit.

The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?

જવાબ : Given: Distance of far point = 80 cm, P= ?
For viewing far-off objects, the focal length of corrective lens,

A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type of the corrective lens used to restore proper vision?

જવાબ : A concave lens of focal length 1.2 m should be used to restore proper vision.

A student has difficulty reading the blackboard while sitting in the last row. What could be the defect the child is suffering from? How can it be corrected? [2012]

જવાબ : The child is suffering from myopia. It can be corrected by using spectacles with concave lenses of suitable focal length.

What is the near point and the far point of a normal human eye? [2011, 2012, 2014]

જવાબ : Near point is 25 cm and far point is infinity.

What is meant by power of accommodation of the eye? [2010,2012,2014,2015]

જવાબ : The images of objects at different distances from the eye are brought to focus on retina by changing the focal length of the eyelens. This is known as power of accommodation of the eye.

Why does the sky appear dark of blue to an astronaut?

જવાબ : At such huge heights due to absence of atmosphere, no scattering of the light takes place. Therefore sky appears dark.

Why does the sun appear reddish early in the morning?

જવાબ : At sunrise, the sun looks almost reddish because only red colour which is least scattered is received by our eye and appears to come from sun. Hence the appearance Of sun at sunrise, near the horizon looks almost reddish.

Explain why the planets do not twinkle ?

જવાબ : Planets being close to earth appear larger in size. A planet can be Considered as a collection of large number of small sized objects. Twinkling effect Of these objects cancel each other. so, planets do not appear to twinkle.

Why do stars twinkle?

જવાબ : The stars twinkle at night, because the star light reaching Our eyes increases and decreases continuously due to atmospheric refraction. When star light reaching our eyes increases, the star looks bright and when the star light reaching our eyes decreases, it appears dim.

A student has difficulty reading the blackboard while sitting in the last row. What could be the defect the child is suffering from ? How can it be corrected ?

જવાબ : As the child has difficulty in reading the blackboard, he is suffering from myopia or short sightedness. To correct this defect, he has to use spectacles with concave lens of suitable focal length.

What is the far point and near point of the human eye with normal vision ?

જવાબ : For human eye with normal vision, far point is at infinity and near point is at 25 cm from the eye.

What is meant by power of accommodation of the eye?

જવાબ : The power Of accommodation of the eye is the ability of the eye to observe the distinct objects clearly which are situated at a large distance from the eye. The ciliary muscles are responsible to change the focal length Of the eye lens. The value of the power of accommodation Of the normal human eye is (d = 25 cm) = 100/f = 100/d = 100/25 = 4 dioptres. The value of power of accommodation Of human eye is about 4D

Why does the sky appear dark instead of blue to an astronaut ?

જવાબ : As an astronaut moves away from the atmosphere of earth, the atmosphere becomes thin. Due to the absence of molecules (or dust particles) in air, the scattering of light does not take place. Thus, sky appears dark in the absence of scattering.

Why does the sun appear reddish early in the morning ?

જવાબ : The light coming from the sun passes through various denser layers of air in the earth’s atmosphere before reaching our eyes near the horizon. Most of the part of blue light and light of small wavelength gets scattered by dust particles near the horizon. So, the light reaching our eyes is of large wavelength. Due to this the sun appears reddish at the time of sunrise and sunset.

Explain why the planets do not twinkle ?

જવાબ : The planets are much nearer to the earth than stars and because of this they can be considered as large source of light. If a planet is considered to be a collection of a very large number of point sources of light, then the average value of change in the amount of light entering the eye from all point size light sources is zero. Due to this the effect of twinkling is nullified.

What happens to the image distance in the eye when we increase the distance of an object from the eye ?

જવાબ : The eye lens of a normal eye forms the images of objects at various distances on the same retina. Therefore, the image distance in the eye remains the same.

Why is a normal eye not able to see clearly the objects placed closer than 25 cm ?

જવાબ : At distance less than 25 cm, the ciliary muscles cannot bulge the eye lens any more, the object cannot be focused on the retina and it appears blurred to the eye, as shown in the given figure.

The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem ?

જવાબ : The remedial lens should make the objects at infinity appear at the far point.
Therefore, for object at infinity, u = ∞
Far point distance of the defected eye, ν = – 80 cm

Negative sign shows that the remedial lens is a concave lens.

A person needs a lens of power -5.5 dioptres for correcting his distant vision. For correcting his near vision he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision ?

જવાબ : (i) Power of distant viewing part of the lens, P1 = -5.5 D
Focal length of this part, f1 = 1p1 = 1−5.5 m = -0.182 m = -18.2 cm
(ii) For near vision,

A student has difficulty reading the blackboard while sitting in the last row. What could be the defect the child is suffering from ? How can it be corrected?

જવાબ : The child is suffering from myopia. The child should use concave lens of suitable focal length

What is the far point and near point of the human eye with normal vision ?

જવાબ : For a human eye with normal vision the far point is at infinity and near point is 25 cm from the eye.

A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type of the corrective lens used to restore proper vision ?

જવાબ : Concave lens.

What is meant by power of accommodation of the eye ?

જવાબ : The power of accommodation of the eye is the maximum variation of its power for focusing on near and far (distant) objects.

Why does the sky appear dark instead of blue to an astronaut

જવાબ : The sky appears dark instead of blue to an astronaut, as scattering of light does not take place outside the earth’s atmosphere.

Why does the Sun appear reddish early in the morning?

જવાબ : White light coming from the sun has to travel more distance in the atmosphere before reaching the observer. During this, the scattering of all colored lights except the light corresponding to red color takes place and so, only the red colored light reaches the observer. Therefore, the sun appears reddish at sunrise and sunset.

Explain why the planets do not twinkle?

જવાબ : Unlike stars, planets don’t twinkle. Stars are so distant that they appear as pinpoints of light in the night sky, even when viewed through a telescope. Since all the light is coming from a single point, its path is highly susceptible to atmospheric interference (i.e. their light is easily diffracted).

Why do stars twinkle?

જવાબ : The twinkling of a star is due to atmospheric refraction of starlight. The starlight, on entering the earth’s atmosphere, undergoes refraction continuously before it reaches the earth. The atmospheric refraction occurs in a medium of gradually changing refractive index.

What happens to the image distance in the eye when we increase the distance of an object from the eye?

જવાબ : The image is formed on the retina even on increasing the distance of an object from the eye. The eye lens becomes thinner and its focal length increases as the object is moved away from the eye.

Why is a normal eye not able to see clearly the objects placed closer than 25 cm?

જવાબ : A normal eye is not able to see the objects placed closer than 25 cm clearly because the ciliary muscles of the eyes are unable to contract beyond a certain limit.

A student has difficulty reading the blackboard while sitting in the last row. What could be the defect the child is suffering from? How can it be corrected?

જવાબ : The student is suffering from short-sightedness or myopia. Myopia can be corrected by the use of concave or diverging lens of an appropriate power.

What is the far point and near point of the human eye with normal vision?

જવાબ : The minimum distance of the object from the eye, which can be seen distinctly without strain is called the near point of the eye. For a normal person’s eye, this distance is 25 cm. The far point of the eye is the maximum distance to which the eye can see objects clearly. The far point of a normal person’s eye is infinity.

A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type of corrective lens used to restore proper vision?

જવાબ : An individual with a myopic eye should use a concave lens of focal length 1.2 m so that he or she can restore proper vision.

What is meant by power of accommodation of the eye?

જવાબ : The ability of the lens of the eye to adjust its focal length to clearly focus rays coming from distant as well from a near objects on the retina, is known as the power of accommodation of the eye.

The phenomenon of scattering of light by colloidal particles gives rise to …………..

જવાબ : Tyndall effect

The splitting of light into its component colours is called …………..

જવાબ : Dispersion

A person with ………….. can see nearby objects clearly but cannot see distant objects clearly.

જવાબ : Myopia

The ability of the eye lens to adjust its focal length is called …………..

જવાબ : Accomodation

. ………….. regulates and controls the amount of light entering the eye.

જવાબ : Pupil

A person needs a lens of power -5.5 dioptres for correcting his distant vision. For correcting his near vision he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision?

જવાબ : The power (P) of a lens of focal length f is given by the relation

Power (P) = 1/f

(i) Power of the lens (used for correcting distant vision) = – 5.5 D

Focal length of the lens (f) = 1/Pf= 1/-5.5 = -0.181 m

The focal length of the lens (for correcting distant vision) is – 0.181 m.

(ii) Power of the lens (used for correcting near vision) = +1.5 D

Focal length of the required lens (f) = 1/P

= 1/1.5 = +0.667 m

The focal length of the lens (for correcting near vision) is 0.667 m.

The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?

જવાબ : The individual is suffering from myopia. In this defect, the image is formed in front of the retina. Therefore, a concave lens is used to correct this defect of vision.

Object distance (u) = infinity = ∞

Image distance (v) = – 80 cm

Focal length = f

According to the lens formula,

A concave lens of power – 1.25 D is required by the individual to correct his defect.

What is short-sightedness ? List two causes for development of short-sightedness. Describe with a ray diagram, how this defect may be corrected using spectacles. (CBSE 2011, 2012)
Or
What is myopia ? State the two causes of myopia with a labelled ray diagram show
(i) the eye defect myopia,
(ii) correction of myopia using lens. [CBSE (All India) 2009, 2015]

જવાબ : A human eye is myopic if it can see the near objects clearly but unable to see far off objects or distant objects clearly.
Correction of short sightedness or Myopia
The image of a distant object (i.e. at infinity) is formed in front of the retina of eye suffering from myopia as shown in figure 5(a). As the image of the object lying at infinity is not formed on the retina of the eye, so such object can not be seen clearly by the Myopic eye. The far point of such an eye is near to the eye as shown in figure 5(b).

This defect can be corrected by using a concave lens of suitable focal length (or power). So, a man suffering from this defect wears spectacles having concave lens of suitable focal length. The concave lens used diverges the rays of light entering the eye from infinity. Hence this lens makes the rays of light appear to come from the far point (O’) of the defective eye as shown in figure 5(c).
Causes of Wlyopia: This defect arises due to either by

1. the elongation of the eye ball or
2. the excessive curvature of the cornea.

Write different parts of eye and explain their functions. Also explain, how an image of an object is formed on the retina of eye. (CBSE 2012)

જવાબ : The main parts of an eye and their functions are given below :

1. Cornea is the outermost part of the eye. It is transparent part of eye and allows the light to enter in the eye.
2. Iris: It is a circular dark diaphragm having a hole in its centre. This hole is called pupil. The circular dark diaphragm has muscles and coloured pigments. The colour of an eye depends upon the colour of these pigments.
The function of iris is to control the size of the pupil. On the other hand, pupil controls and regulates the light entering the eye. The pupil becomes small when bright light falls on the eye. However, it becomes wide when there is dim light.
3. Lens: The eye lens is a crystalline double convex lens and made of transparent and flexible tissues. It is behind the pupil and held by the muscles called ciliary muscles. It focuses the images of objects on the retina of the eye.
4. Ciliary muscles: These muscles hold the eye lens in position. Ciliary muscles controls the focal length of the eye lens. When these muscles contracts, then the lens becomes thick and the focal length of the lens decreases. On the other hand, when ciliary muscles are relaxed, then the lens becomes thin and the focal length of the lens increases.
5. Retina: It acts as a light-sensitive screen to obtain the image of the object. It contains number of cells in the form of rods and cones which are sensitive to light. These cells converts light energy into electrical impulses or signals.
6. Optic nerve: Optic nerve is formed by the nerve fibres coming from the retina. It carries nerve or electrical impulses or signals to the brain. The brain finally interprets the signal.

State the difference in colours of the sun observed during sunrise/sunset and noon. Give explanation for each.
(CBSE 2013)

જવાબ : The colour of the sun observed during sunrise/sunset is reddish, while its colour observed at noon is white. At sunrise/sunset, the sun is very far away from the observer. So, the sunlight travels longer distance through the atmosphere to reach the eyes of the observerer. The scattering of blue light is more than the scattering of red light. So, more red light reaches our eye and hence sun appears reddish.
During noon the sun is overhead and light from the sun travels less distance through air to reach our eye. In this case, scattering of all colours is very small. Hence, the sun appears white.

Why are stop signals on roads in red coloured light ?
[CBSE (Delhi) 2008, CBSE (All India) 2009, 2012, 2015]

જવાબ : When light falls on the signal, all colours are scattered much more than that of red colour. So the red colour suffering least scattering remains confined around the signal, which in turn illuminates the signal significantly. Thus, the danger signals can be seen from very far off distances. Moreover, among all colours, red colour or red light is scattered least by smoke or fog. Hence, red signals are visible even through the smoke or fog.

Why is the colour of clear sky blue ? Explain. (CBSE 2012, 2014, 2015, 2016)
Or
Why does sky look blue on a clear day ?
[CBSE (Delhi) 2008, 2011]

જવાબ : When sunlight enters the earth’s atmosphere, the atoms or molecules of the gases present in the atmosphere scatter this light. Since wavelength of red colour is larger than the wavelengths of other colours in sunlight, so red colour is scattered least. Violet colour is scattered the most followed by blue, green, yellow, orange and red colours respectively. Our eye is more sensitive to the blue light than the violet light. Therefore, scattered light in the sky contains blue colour in plenty and hence the clear sky appears blue.
Note : Sky appears greyish over cities having industrial units. The smoke and dust particles in the atmosphere over such cities scatter red, orange and yellow colours more than other colours of small wavelengths. Hence, the sky appears greyish.

Why does the sun appear oval at sunset and sunrise but appears circular at noon ?

જવાબ : The sun appears oval (or flattened) at sunset and sunrise but appears circular at noon
At sunset and sunrise, the sun is near the horizon. The rays of light from the upper and lower edge of the sun bend unequally while travelling through the atmosphere. As a result of this phenomenon, the sun appears oval or flattened (Figure 15).

At noon, the sun is overhead. The rays of light from the sun enter the atmosphere normally and hence they do not bend at all while passing through the atmosphere. Therefore, the sun appears circular at noon.

We can see the sun for few minutes even after it has actually set. Explain, why ?
Or
Explain with the help of diagram why the sun is visible to us two minutes before the actual sun-rise and two minutes after the sunset. (CBSE 2012, 2014)

જવાબ : We can see the sun for few minutes even after it has actually set
Actual sun sets when it is below the horizon in the evening. The rays of light from the sun below the horizon reach our eyes because of refraction of light through atmosphere. These rays appear to come from the apparent position of the sun which is above the horizon as shown in figure 14.

Hence we can see the sun for few minutes (about 2 minutes) even after it has actually set. Similarly, the sun can be seen about 2 minutes before it actually rises. Thus, we gain about 4 minutes of additional daylight each day.

Describe atmospheric refraction. Explain with the help of diagram why the sun is visible to us two minutes before the actual sun-rise and two minutes even after the sunset. (CBSE 2015, CBSE (Foreign) 2017)

જવાબ : The refraction of light takes place, when light travels from one medium to another medium. In other words, refraction of light takes place, when light goes from an optically rarer medium to an optical denser medium or vice-versa. The earth’s atmosphere has air all around. The air in the atmosphere is in the form of layers. The different layers of air have different densities. As we go higher and higher, the density of the layers of air goes on decreasing. Thus, the layers of air close to the surface of the earth have more density than the density of the air layers far away from the surface of the earth. Therefore, the layers of air close to the surface of the earth behave as optically denser medium and the layers of air far away from the surface of the earth behave as optically rarer medium.
When sun light enters the earth’s atmosphere, it continuously goes from rarer to the denser medium and hence refraction of light takes place. The refraction of light taking place in the atmosphere is known as atmospheric refraction.

Why do we see a rainbow in the sky only after rainfall ? [NCERT Question Bank]

જવાબ : You must have seen a beautiful rainbow in the sky after rainfall. This rainbow is formed due to the dispersion of sunlight.
When sunlight falls on the water drops suspended in the atmosphere after rainfall, rainbow is formed due to the dispersion of sun light. The water drops suspended in air (or atmosphere) act as prisms.
Rainbow is the example of dispersion of sunlight. The formation of rainbow after rainfall is shown in figure 12.

Rainbow is observed during a rainfall or after the rainfall or when we look at a water fountain provided the sun is at the back of the observer.
Conditions for Observing a Rainbow
When sunlight falls on a water drop suspended in air, then the sunlight is refracted. The refracted sunlight splits (or dispersed) into its constituent colours (i.e., seven colours). Thus, water drop suspended in air behaves as a glass prism. The red colour deviates the least and the violet colour deviates the most. Different colours of refracted sunlight fall on the opposite face of the water drop.- Now, each colour suffers internal reflection. The reflected colours on reaching the lower surface of water drop are refracted again into the air. Thus, we get a spectrum of seven colours, which is known as a rainbow.

Based on Atmospheric Refraction of light

How did Newton, using two identical glass prisms, show that white light is made of seven colours ? (CBSE 2017)

જવાબ : Investigate dispersion of white light in a glass prism

1. Place a thick card board having a small hole in it in front of a glass prism.
2. Allow sunlight to fall on the card board. The hole in the card board allows a beam of white light to fall on the prism.
3. Place a white screen at some distance away from the prism.
4. Rotate the prism slowly until a band of seven colour is figure 9 observed on the white screen.
Conclusion : “When a beam of white light falls on a glass prism, white light splits into its constituent colours (i.e., seven colours). This phenomenon of splitting white light into seven colours by a glass prism is known as dispersion of white light.

How will you use two identical glass prisms so that a narrow beam of white light incident on one prism emerges out of the second prism as a beam of white light ? Draw a labelled diagram to illustrate it. (CBSE 2015)

જવાબ : Perform an activity to show that the colours of white light splitted by a glass prism can be recombined to get white light by another glass prism.
Apparatus required: Two glass prisms made of same kind of glass, a card board having a fine hole at its centre, a white screen.
Procedure:

1. Place a card board in front of a prism A. A ray of white light coming from the hole in the card board falls on the prism A (Figure 11).
2. White light splits into seven colours by prism A is made to fall on another glass prism B placed with its base upward. Since prism A disperses white light, so it is known as dispersing prism.
3. The prism B deviates colours of light towards its base. The various colours recombines at the opposite lace of glass prism B.
4. This activity was initially performed by the great scientist Issac Newton.
The light received on the white screen placed in front of prism B is white. Since the prism B recombines the colours of light to produce white light, so glass prism B is known as recombination prism.

Why do different colours get separated when white light passes through prism I How can we recombine the components of white light after a prism has separated them ? Explain with the help of figure. (CBSE 2012)

જવાબ : Different colours have different speeds in glass prism. Due to this fact, different colours get separated while passing through the prism.
Perform an activity to show that the colours of white light splitted by a glass prism can be recombined to get white light by another glass prism.
Apparatus required: Two glass prisms made of same kind of glass, a card board having a fine hole at its centre, a white screen.
Procedure:

1. Place a card board in front of a prism A. A ray of white light coming from the hole in the card board falls on the prism A (Figure 11).
2. White light splits into seven colours by prism A is made to fall on another glass prism B placed with its base upward. Since prism A disperses white light, so it is known as dispersing prism.
3. The prism B deviates colours of light towards its base. The various colours recombines at the opposite lace of glass prism B.
4. This activity was initially performed by the great scientist Issac Newton.
The light received on the white screen placed in front of prism B is white. Since the prism B recombines the colours of light to produce white light, so glass prism B is known as recombination prism.

When white light passes through a glass prism, seven colours namely red, orange, yellow, green, blue, indigo and violet are seen on the white screen. All these colours have different angles of deviation. Explain why ?

જવાબ : Speed of colour in a medium depends upon its wavelength. All the colours have different wavelength. The red colour has the longest wavelength and violet colour has the last wavelength. Therefore, red colour has the highest speed in the glass prism and the violet colour has the lowest speed in the glass prism. Hence, all colours of white light are refracted by different amounts while passing through the glass prism. Therefore, all the colours have different angles of deviations.

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. (CBSE 2017)

જવાબ : Name of defet : Presbyopia
Type of lens required by persons suffering presbyopia to improve the vision is bi-focal lens.
Bi-focal lens consists of a concave lens which forms the upper surface of a bi-focal lens and a convex lens which forms the lower surface of the bi-focal lens. The upper surface of the lens enables the person to see distant objects clearly and the lower surface of the lens enables the person to see the near objects clearly.

An old person is unable to see clearly nearby object as well as distant objects.

1. What defect of vision is he suffering from ?
2. What kind of lens will be required to see clearly the nearby as well as distant objects ? Give reasons ?
(CBSE 2012, 2014)

જવાબ :

1. Presbyopia.
2. This defect is connected by using bi-focal lens. A bi-focal lens consists of a concave lens which forms the upper surface of lens and a convex lens which form the lower surface of the lens. The upper surface enables the person to see distant objects clearly and the lower surface helps the person to see the near objects clearly.

What is presbyopia ? State the cause of Presbyopia. How is presbyopia of a person be corrected ?
[CBSE (All India) 2009]

જવાબ : A human eye which cannot see the near objects and distant objects clearly is said to suffer from a defect known as Presbyopia. Eye suffering from Presbyopia cannot read and write comfortably.
This defect arises due to the ageing of a person. The ciliary muscles are weakened and the flexibility of the crystalline lens of the human eye decreases with age of the person. As a result, human eye is unable to focus on close as well as distant objects.
This defect can be corrected by using a bi-focal lens. A bi-focal lens consists of a concave lens which forms the upper surface of the bi-focal lens and a convex lens which forms the lower surface of the bi-focal lens. The upper surface of bi-focal lens (i.e. the concave lens) enables the person to see distant objects clearly and the lower surface of bi-focal lens {i.e. convex lens) enables the person to see the near objects.

What is long-sightedness ? How can this defect be corrected ?

જવાબ : A human eye which can see far off objects or distant objects clearly but can not see the near objects clearly is said to be suffered with a defect known as long sightedness or far sightedness or Hypermetropia.
Causes of Hypermetropia: This defect arises due to either by

1. the increase in the focal length of eye lens or
2. the size of the eye ball becomes too small so that the light rays from the nearby points or objects are not brought to focus on the retina of the eye.

What is short-sightedness ? How can this defect be corrected ? (Bihar Board 2012)

જવાબ : Short-sightedness or near-sightedness or Myopia
A human eye is myopic if it can see the near objects clearly but unable to see far off objects or distant objects clearly.
Causes of Wlyopia: This defect arises due to either by

1. the elongation of the eye ball or
2. the excessive curvature of the cornea.

How is a normal eye able to see distinctly distant as well as nearer objects ? What is the distance of distinct vision ? (CBSE 2012)

જવાબ : Eye has the ability known as accommodation of eye to see distant as well as nearer objects clearly. When objects is far away, the focal length of lens is increased due to the relaxed ciliary muscles. Hence sharp image of object is formed on the retina of eye. When object is nearer to the eye, the focal length of lens is decreased due to the contraction of ciliary muscles and hence sharp image of the object is formed on the retina of eye. The distance of distinct vision is 25 cm.

What is meant by scattering of light ? (CBSE 2014, 2015, 2016)

જવાબ : When sunlight enters the atmosphere of the earth, the atoms and molecules of different gases present in the atmosphere absorb this light. Then these atoms and molecules of the gases re-emit light in all directions. This process is known as scattering of light. The atoms or particles scattering light are known as scatterers.
The intensity of scattered light is inversely proportional to the fourth power of the wavelength of incident light, if the size of the particles (say atoms or molecules) scattering the light is less than the wavelength of the incident light.

A convex lens made of glass forms a sharp image on the screen for a particular position of an object with respect to the lens. A human eye lens is also a convex lens but it can form sharp images on the retina of eye for different positions of the objects. Explain, why ? (CBSE 2010)

જવાબ : A convex lens made of glass has a fixed focal length and hence it forms a sharp image on the screen for a particular position of an object. However, the focal length of human eye lens can be changed by the action of ciliary muscles. In other words, human eye lens has the ability to chartge its focal length to form sharp images of objects at different positions. The process is known as accommodation of eye.

Why does the sun appear reddish early in the morning?  [2011,2012]

જવાબ : At the time of shnrise, the rays of the sun have to travel a larger atmospheric distance. As the wavelength of red colour is the largest of all the colours of sunlight, most of the blue colour and other colours are scattered away. Only red . colour which is,,least scattered, enters into our eyes. Hence, the sun appears reddish at the time of sunrise.

Make a diagram to show how hypermetropia is corrected. The near point of a hypermetropic eye is 1 m. What is the power of the lens required to correct this defect? Assume that the near point of the normal eye is 25 cm. [2011]

જવાબ : The required diagram is shown below:

Make a diagram to show how hypermetropia is corrected. The near point of a hypermetropic eye ¡s 1 m. What ¡s the power of a lens required to correct this defect? Assume that near point of the normal eye is 25 cm.

જવાબ :

The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?

જવાબ :

A person needs a lens of power – 5.5 dioptres for correcting his distant vision. For correcting his near vision he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision?

જવાબ :

A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type of the corrective lens used to restore proper vision?

જવાબ : The far point for myopic eye is 1.2m.

Why do stars twinkle ?

જવાબ : Stars appear to twinkle due to atmospheric refraction. The light of star after the entry of light in earth’s atmosphere undergoes refraction continuously till it reaches the surface of the earth. Stars are far away. So, they are the point source of light. As the path of light coming from stars keep changing, thus the apparent position of stars keep changing and amount of light from stars entering the eye keeps twinkling. Due to which a star sometimes appear bright and sometimes dim, which is the effect of twinkling.

Make a diagram to show how hypermetropia is corrected. The near point of a hypermetropic eye is 1 m. What is the power of the lens required to correct this defect? Assume that the near point of the normal eye is 25 cm.

જવાબ : An individual suffering from hypermetropia can see distinct objects clearly but he or she will face difficulty in clearly seeing objects nearby. This happens because the eye lens focuses the incoming divergent rays beyond the retina. This is corrected by using a convex lens. A convex lens of a suitable power converges the incoming light in such a way that the image is formed on the retina, as shown in the following figure.

The convex lens creates a virtual image of a nearby object (N’ in the above figure) at the near point of vision (N) of the individual suffering from hypermetropia.

The given individual will be able to clearly see the object kept at 25 cm (near point of the normal eye), if the image of the object is formed at his near point, which is given as 1 m.

Object distance, u= – 25 cm

Image distance, v= – 1 m = – 100 m

Focal length, f

Using the lens formula,

A convex lens of power +3.0 D is required to correct the defect.

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Match the following columns

 Column I Column II (a) Rainbow formation (i) A person cannot see nearby objects clearly (b) Twinkling of stars (ii) Regulates and controls amount of light entering the eye (c) Blue colour of sky (iii) Scattering of light (d) Myopia (iv) Dispersion (e) Hypermetropia (v) Infinity (f) Least distance of distinct vision (vi) Membrane which controls the size of pupil (g) Far point of eye (vii) Atmospheric refraction (h) Iris (viii) Twenty five centimeters (i) Pupil (ix) A person cannot see distant objects distinctly

જવાબ :

 Column I Column II (a) Rainbow formation (iv) Dispersion (b) Twinkling of stars (vii) Atmospheric refraction (c) Blue colour of sky (iii) Scattering of light (d) Myopia (ix) A person cannot see distant objects distinctly (e) Hypermetropia (i) A person cannot see nearby objects clearly (f) Least distance of distinct vision (viii) Twenty five centimeters (g) Far point of eye (v) Infinity (h) Iris (vi) Membrane which controls the size of pupil (i) Pupil (ii) Regulates and controls amount of light entering the eye

 a. Lens used for correction of myopia Spectrum b. The phenomenon of scattering of light by colloidal particles . Infinity c. . The band of coloured components of a light beam Pupil d. Part of eye which regulates and controls the amount of light entering the eye Concave lens e. Lens used in correction of presbyopia Red f. Lens used in correction of hypermetropia Convex lens g. Colour used for danger signal Tyndall effect h. Farthest point for a normal eye Bifocal lens

જવાબ :

a. Concave lens
b. Tyndall effect
c. Spectrum
d. Pupil
e. Bifocal lens
f. Convex lens
g. Red
h. Infinity

 a. Eye defect in which a person cannot see distant objects distinctly Presbyopia b. Eye defect in which a person cannot be nearby objects clearly Twenty five c. Splitting of light into its component colours Hypermetropia d. Least distance of distinct vision (in cm) Myopia e. The eye defect which is caused by weakening of ciliary muscles Dispersion

જવાબ :

a. Myopia
b. Hypermetropia
c. Dispersion
d. Twenty five
e. Presbyopia