NCERT Exemplar Solution Class 10 Science Chapter 11
NCERT Exemplar Solution Class 10 Science Chapter 11 The Human Eye and the Colorful World all Questions Answer Solution. NCERT Exemplar Class 10 Science Chapter 11 Solution PDF.
NCERT Exemplar Solution Class 10 Science Chapter 11: Overview
NCERT Exemplar Solution Class 10 Science Chapter 11 |
|
Board |
NCERT |
Topic |
Exemplar Problem Solution |
Class |
10 |
Subject |
Science |
Chapter |
11 |
Chapter Name |
The Human Eye and the Colorful World |
NCERT Exemplar Solution Class 10 Science Chapter 11 The Human Eye and the Colorful World
Multiple Choice Questions:
1) A person cannot see distinctly objects kept beyond 2 m. This defect can be corrected by using a lens of power
(a) + 0.5 D
(b) – 0.5 D
(c) + 0.2 D
(d) – 0.2 D
Ans: (b) – 0.5 D
The person cannot see distinctly objects kept beyond 2m, means the person is suffering from myopia.
Myopia is the disease of near-sightedness i.e. that person can see the nearby objects clearly but he cannot see the distant objects distinctly.
This defect is covered by using concave lens which is having negative focal length.
Given that, person cannot see beyond 2m.
Hence, Power of lens = – 1/focal length
P = -1/2
P = -0.5 diopters
2) A student sitting on the last bench can read the letters written on the blackboard but is not able to read the letters written in his text book. Which of the following statements is correct?
(a) The near point of his eyes has receded away
(b) The near point of his eyes has come closer to him
(c) The far point of his eyes has come closer to him
(d) The far point of his eyes has receded away
Ans: (a) The near point of his eyes has receded away
Since, the student is suffering from hypermetropia in which the person can see distant objects clearly because his near point moves away beyond 25cm.
But he cannot see nearby objects clearly because his near point is farther away from the normal near point.
3) A prism ABC (with BC as base) is placed in different orientations. A narrow beam of white light is incident on the prism as shown in Figure 11.1. In which of the following cases, after dispersion, the third color from the top corresponds to the color of the sky?
(a) (i)
(b) (ii)
(c) (iii)
(d) (iv)
Ans: (b) (ii)
If we placed the prism as shown in fig.a) i.e. base is below then we will get the spectrum of light such as red, orange, yellow & so on.
But if we placed the prism inverted i.e. the base is in upward as in fig.b) direction then the spectrum obtained will be of the form violet, indigo & third color from the top becomes blue which corresponds to the color of sky.
4) At noon the sun appears white as
(a) Light is least scattered
(b) All the colors of the white light are scattered away
(c) Blue color is scattered the most
(d) Red color is scattered the most
Ans: (a) Light is least scattered
At noon the sun is at overhead, so the light coming from the sun travels shorter distance in atmosphere.
And at that time only a small amount of blue and violet colored light are scattered, due to which sun appears white at noon.
5) Which of the following phenomena of light are involved in the formation of a rainbow?
(a) Reflection, refraction and dispersion
(b) Refraction, dispersion and total internal reflection
(c) Refraction, dispersion and internal reflection
(d) Dispersion, scattering and total internal reflection
Ans: (c) Refraction, dispersion and internal reflection
The phenomenon of formation of rainbow is natural & it forms after raining.
The small droplets of water acts like small prisms & the sunlight get refracted, dispersed and then internally reflected after we see the spectrum of seven colors on the opposite side to the sun.
6) Twinkling of stars is due to atmospheric
(a) Dispersion of light by water droplets
(b) Refraction of light by different layers of varying refractive indices
(c) Scattering of light by dust particles
(d) Internal reflection of light by clouds
Ans: (b) Refraction of light by different layers of varying refractive indices
The twinkling of stars is due to atmospheric refraction of starlight by different layers of varying refractive indices.
When starlight enters into the earth’s atmosphere, it undergoes continuous refraction before reaching to earth’s atmosphere due to different refractive indices of layers.
7) The clear sky appears blue because
(a) Blue light gets absorbed in the atmosphere
(b) Ultraviolet radiations are absorbed in the atmosphere
(c) Violet and blue lights get scattered more than lights of all other colors by the atmosphere (d) light of all other colors is scattered more than the violet and blue color lights by the atmosphere
Ans: (c) Violet and blue lights get scattered more than lights of all other colors by the atmosphere
The blue color of the sky is due to scattering of light by molecules of air and dust particles whose size is very small as compared to wavelength of light.
The wavelength of blue & violet light is small & their scattering is more than other colors.
Hence, the clear sky appears blue.
8) Which of the following statements is correct regarding the propagation of light of different colors of white light in air?
(a) Red light moves fastest
(b) Blue light moves faster than green light
(c) All the colors of the white light move with the same speed
(d) Yellow light moves with the mean speed as that of the red and the violet light
Ans: (a) Red light moves fastest
Since, the wavelength of light decreases from red to violet, so the velocity of light also decreases from red to violet. Hence re light moves fastest.
The order is, λv<λy<λr and Vv<Vy<Vr
9) The danger signals installed at the top of tall buildings are red in color. These can be easily seen from a distance because among all other colors, the red light
(a) Is scattered the most by smoke or fog
(b) Is scattered the least by smoke or fog
c) Is absorbed the most by smoke or fog
(d) Moves fastest in air
Ans: (b) Is scattered the least by smoke or fog
The danger signal installed at the top of tall buildings are red in color.
Because red color is least scattered by the smoke or fog, due to which it can be easily seen in the same color at a distance.
10) Which of the following phenomena contributes significantly to the reddish appearance of the sun at sunrise or sunset?
(a) Dispersion of light
(b) Scattering of light
(c) Total internal reflection of light
(d) Reflection of light from the earth
Ans: (b) Scattering of light
The sun appears reddish at the time of sunrise or sunset because of scattering of light.
At sunset & sunrise, the blue and violet colored are scattered away already & hence the scattered light reaching to our eyes is only red. So sun appears reddish at that time.
11) The bluish color of water in deep sea is due to
(a) The presence of algae and other plants found in water
(b) Reflection of sky in water
(c) Scattering of light
(d) Absorption of light by the sea
Ans: (c) Scattering of light
According to Rayleigh’s scattering,
Scattering of light α 1/ λ4
Hence, less the wavelength of light more is the scattering.
The order is, λv<λy<λr and hence other colors get absorbed by the water and only more scattered light by water is blue. Hence the water of deep sea appears blue due to scattering of light.
12) When light rays enter the eye, most of the refraction occurs at the
(a) Crystalline lens
(b) Outer surface of the cornea
(c) Iris
(d) Pupil
Ans: (b) Outer surface of the cornea
Light enters into the eye through a thin membrane which is called as cornea.
Cornea is the front part of the eyeball which forms transparent bulge.
When light enters cornea via air there is huge change in refractive index, so that most of the refraction occurs at outer front surface of eyeball called as cornea.
13) The focal length of the eye lens increases when eye muscles
(a) Are relaxed and lens becomes thinner
(b) Contract and lens becomes thicker
(c) Are relaxed and lens becomes thicker
(d) Contract and lens becomes thinner
Ans: (a) Are relaxed and lens becomes thinner
When curvature of the eye lens changes its focal length also changes.
When the eye muscles get relaxed and lens become thinner, the focal length get increased due to which we can see the distant object clearly.
14) Which of the following statement is correct?
(a) A person with myopia can see distant objects clearly
(b) A person with hypermetropia can see nearby objects clearly
(c) A person with myopia can see nearby objects clearly
(d) A person with hypermetropia cannot see distant objects clearly
Ans: (c) A person with myopia can see nearby objects clearly
Myopia is the disease of near-sightedness i.e. that person can see the nearby objects clearly but he cannot see the distant objects clearly.
Short Answer Questions:
15) Draw ray diagrams each showing
Ans: (i) Myopic eye
a)Fig. showing far point of myopic eye
b) Fig. showing myopic eye
(ii) Hypermetropic eye
Fig. showing near point of hypermetropic eye
Fig. showing hypermetropic eye
16) A student sitting at the back of the classroom cannot read clearly the letters written on the blackboard. What advice will a doctor give to her? Draw ray diagram for the correction of this defect.
Ans:
The student sitting at the back of the classroom cannot read clearly the letters written on the blackboard that means she is having the defect called as myopia.
Myopia is the disease of near-sightedness i.e. that person can see the nearby objects clearly but she cannot see the distant objects clearly.
Because her far point lies nearer than infinity. Due to which the image is formed in front of retina instead of on the retina. So she will see the objects only up to a few meters.
A doctor will give advice to her, to use concave lens of suitable power to form the image back on to the retina.
Fig. showing myopic eye
Following fig. shows the correction for myopia with a concave lens.
17) How are we able to see nearby and also the distant objects clearly?
Ans:
The eye lens is made up from a jelly-like, fibrous material. And the ciliary muscles can modify its curvature to some extent.
As the curvature of eye lens changes it leads to change in its focal length.
When this muscles get relaxed, the lens becomes thinner due to which its focal length get increased which results in seeing the distant objects clearly.
In contradiction to this, when we see the objects near to our eye; the ciliary muscles get contracted due to which curvature of lens increases.
And the eye lens becomes thicker, and hence focal length of eye lens decreases, which leads to see nearby objects clearly.
To see any object comfortably and clearly we have to hold it at a distance of 25cm, which is called as least distance of distinct vision or near point of the eye.
18) A person needs a lens of power –4.5 D for correction of her vision.
Ans:
(a)What kind of defect in vision is she suffering from?
Since the person needs a lens of power -4.5D, hence focal length is negative.
So the lens used is concave lens.
And concave lens is used only for the correcting the defect called as myopia.
Hence the person is suffering from myopia.
(b)What is the focal length of the corrective lens?
Given that: power of lens = -4.5D
Focal length of lens = 1/P = 1/ (-4.5) = -0.22cm
Thus, the corrective lens used has focal length -0.22cm.
(c) What is the nature of the corrective lens?
From above discussion, the corrective lens used is concave lens.
19) How will you use two identical prisms so that a narrow beam of white light incident on one prism emerges out of the second prism as white light? Draw the diagram.
Ans: •Following figure shows the positions of two identical prism so that we will get the white only after emerging from second prism.
Isaac Newton was the first person who used glass prism to obtain spectrum of light.
He passed the white light through glass prism and obtained spectrum of light containing seven colors.
Again he passed this spectrum of light through second identical prism to split this seven colors again, but he was observed that when two identical prisms are placed inverted to each other as shown in fig. above then after emerging light beam from second prism does not contains spectrum of light and it becomes wholly the white light.
From this experiment he conclude that sunlight is made up from seven colors which are V I B G Y O R.
20) Draw a ray diagram showing the dispersi on through a prism when a narrow beam of white light is incident on one of its refracting surfaces. Also indicate the order of the colors of the spectrum obtained
Ans:
Following fig. shows the dispersion through a prism when a narrow beam of light is incident on one of its refracting surface and the order of the colors obtained from bottom to top is
V I B G Y O R.
Fig. Ray diagram showing the dispersion through a prism
21) Is the position of a star as seen by us its true position? Justify your answer.
Ans: The position of the star as seen by us is its apparent position which changes continuously, due to which we see twinkling of stars.
This is due to the refraction of starlight through earth’s atmosphere. The light coming from star undergoes numbers of refraction continuously through earth’s atmosphere due the variation of refractive index in layers of atmosphere.
Due to which the starlight get bends towards normal & the position of star is different from its actual position.
When viewed from the horizon the position of star appears slightly higher than the actual position.
This apparent position of the star does not remains constant it changes continuously due to changes in the condition of atmosphere.
The star is at large distant apart from our eye which acts as a point source of light. The light coming from star reaching to our eyes gets flicker due to which it appears brighter sometime and fainter sometime.
And this is called as twinkling of stars.
explains the twinkling of stars in detail.
22) Why do we see a rainbow in the sky only after rainfall?
Ans: Rainbow is formed after raining because of following reasons:
After rainfall the sky is fully clear and clouds are in the opposite direction to that of sun, so that sunlight is bright.
After raining the atmosphere is filled with small water droplets, and this each drop of water acts as a tiny prism.
When the sunlight is passed through this raindrops at right angle then it splits into spectrum of seven colors which we called as rainbow.
The rainbow is always formed at opposite side of the sun.
23) Why is the color of the clear sky blue?
Ans:
The blue color of the sky is due to scattering of light by molecules of air and dust particles whose size is very small as compared to wavelength of light.
The wavelength of blue & violet light is small & their scattering is more when the light is passed through them than other colors.
According to Rayleigh’s scattering,
Scattering of light α 1/ λ4
Hence, less the wavelength of light more is the scattering.
The order is, λv<λb….<λr and hence the violet & blue colors are scattered more in atmosphere but violet light is not more sensitive to our eyes and we see blue color mostly, so the sky appears blue.
24) What is the difference in colors of the Sun observed during sunrise/sunset and noon? Give explanation for each.
Ans:
The sun appears reddish at the time of sunset or sunrise while appears white at noon.
At the time of sunset or sunrise, the sun is very near to horizon. So it needs to travel longer distance through atmosphere to reach the observer on earth.
During this, the blue and violet light are scattered more and away from the path of light & hence, cannot reach the observer directly. The light reaching to observer is only red light. And hence sun appears reddish at the time of sunset or sunrise.
At noon the sun is at overhead, so the light coming from the sun travels shorter distance in atmosphere.
And at that time only a small amount of blue and violet colored light are scattered, due to which sun appears white at noon.
The diagram below explains the above discussed topic.
Long Answer Questions:
25) Explain the structure and functioning of Human eye. How are we able to see nearby as well as distant objects?
Ans: The following fig. shows the structure of human eye:
Structure and function of human eye:
The all universe we can see only because of the vision of our eyes. If the eyes are closed we cannot see the colorful world.
The structure and function of human eye is like a camera.
The light enters through the cornea which is thin membrane forming bulge at the front part of the eye which is also called as eyeball.
Eyeballs are spherical in shape with diameter of about 2.3 cm.
The most of the refraction is takes place at the outer surface cornea due to large change in refractive index.
The real and inverted image is formed on the retina after refraction.
The crystalline lens adjusts the focal length of lens of eye in order to see the objects clearly and distinctly which are nearby or distant apart.
The small part called as pupil which controls the amount of light entering the eye. And the size of pupil is controlled by muscular part called as iris.
The retina is a delicate part composed of light sensitive cells, when the image is formed it gets activated and produces an electrical impulse.
This electrical impulse reaches to brain via optic nerves.
The brain detect the signal and we see the object.
How are we able to see nearby as well as distant objects?
The eye lens is made up from a jelly-like, fibrous material. And the ciliary muscles can modify its curvature to some extent.
As the curvature of eye lens changes it leads to change in its focal length.
When this muscles get relaxed, the lens becomes thinner due to which its focal length get increased which results in seeing the distant objects clearly.
In contradiction to this, when we see the objects near to our eye; the ciliary muscles get contracted due to which curvature of lens increases.
And the eye lens becomes thicker, and hence focal length of eye lens decreases, which leads to see nearby objects clearly.
To see any object comfortably and clearly we have to hold it at a distance of 25cm, which is called as least distance of distinct vision or near point of the eye.
When do we consider a person to be myopic or hypermetropic? Explain using diagrams how the defects associated with myopic and hypermetropic eye can be corrected?
Ans:
The person cannot see distinctly objects kept beyond 2m, means the person is suffering from myopia.
Myopia is the disease of near-sightedness i.e. that person can see the nearby objects clearly but he cannot see the distant objects distinctly.
The person which see the distant objects clearly but cannot see the nearby objects distinctly and clearly means, the person is suffering from hypermetropia.
Hyepermetropia is also called as disease of far-sightedness. In which the near point of the moves farther away from the normal near point of the eye.
In case of myopia the image is formed in front of retina, because of the elongation of curvature of eye lens as shown in fig a)
This defect is corrected as shown in fig.b) by using concave lens of suitable power and focal length, which brings the image back on to the retina and a person can see the distant objects clearly and distinctly.
fig. a) myopic eye
fig. b) correction for myopia using concave lens
In case of hypermetropia , the focal length of the eye lens increases more due to which the size of eyeballs becomes small.
And hence the image of the object nearby is formed behind the retina as shown in fig.c).
This defect can be corrected as shown in fig.d)by using convex lens of suitable power and focal length due to which the image is formed on the retina and the person can see nearby objects clearly and distinctly.
fig. c) hypermetropic eye
fig. d) correction for hypermetropia using convex lens
27) Explain the refraction of light through a triangular glass prism using a labelled ray diagram. Hence define the angle of deviation.
Ans: The following figure shows the ray diagram for refraction of light through a triangular glass prism:
P: Angle of prism
i: Angle of incidence
r: angle of refraction
e: Angle of emergence
D: Angle of deviation
AB: Incident ray
EF: Refracted ray
FD: Emergent ray
The prism is formed from two triangular and three rectangular bases.
The angle at which two plane rectangular surfaces meet is called as refracting angle of prism.
The fig shows ray diagram for refraction of light through a triangular glass prism.
The refraction of light through triangular glass prism is similar to that of refraction through a rectangular glass slab.
When a ray of light AB enters the prism it travels from air to glass medium i.e. rarer to denser medium, so the refracted ray EF bends towards normal MM’.
Again when this ray EF travels from glass to air medium i.e. denser to rarer medium, and refracted as CD which bends away from the normal NN’ as in fig.
The angle through which all the colored light deviated from prism is called as angle of deviation and it is different for different media.
The different colors of light are travelling in different directions. The red color bends less so its angle of deviation is less and increases towards the violet color.
The different colors of light are due to different wavelengths of colors.
The wavelength of red is more and decreases from red to violet.
Thus, when the white beam of light i.e. sunlight is passed through the prism it get dispersed in seven colors called as spectrum of light.
28) How can we explain the reddish appearance of sun at sunrise or sunset? Why does it not appear red at noon?
Ans:
The sun appears reddish at the time of sunset or sunrise while appears white at noon.
At the time of sunset or sunrise, the sun is very near to horizon. So it needs to travel longer distance through atmosphere to reach the observer on earth.
During this, the blue and violet light are scattered more and away from the path of light & hence, cannot reach the observer directly. The light reaching to observer is only red light. And hence sun appears reddish at the time of sunset or sunrise.
At noon the sun is at overhead, so the light coming from the sun travels shorter distance in atmosphere.
And at that time only a small amount of blue and violet colored light are scattered, due to which sun appears white at noon.
The diagram below explains the above discussed topic.
Fig. showing reddish color of sun at sunset/sunrise and at noon white
29) Explain the phenomenon of dispersion of white light through a glass prism, using suitable ray diagram.
Ans: The following figure shows the suitable ray diagram for dispersion of light through a glass prism:
The phenomenon of spreading of white light into the spectrum of light when passed through the prism is called as dispersion of light.
When the white light is passed through the prism at a certain angle of incidence, it get refracted first and emerge out as a band of seven colors called as spectrum of light as shown in fig.
The order of color is violet to red from top to bottom which is called as VIBGYOR
The angle of deviation for each color is different.
The red color bends less while the violet color get bended more as in fig.
The wavelength of violet light is less and increases towards red.
And hence, the speed of light increases from violet to red also.
The different colors of light are due to the different paths traveled by each light.
When the two prisms are placed inverted to each other and if white light is passed through the first prism, the light coming from second also a white light only.
The phenomenon of formation of rainbow is also the reason of dispersion, where small droplets acts as a tiny prisms.
30) How does refraction take place in the atmosphere? Why do stars twinkle but not the planets?
Ans: In atmosphere there are various layers and their temperature is also varying.
As we go upwards from the land the temperature decreases. The layer having high temperature becomes less dense than the layer having less temperature.
Hence, the hot air has low refractive index than the cold air.
Because of this variation in refractive indices of atmospheric layers, there is refraction of light through atmosphere.
The position of the star as seen by us is its apparent position which changes continuously, due to which we see twinkling of stars.
This is due to the refraction of starlight through earth’s atmosphere. The light coming from star undergoes numbers of refraction continuously through earth’s atmosphere due the variation of refractive index in layers of atmosphere.
Due to which the starlight get bends towards normal & the position of star is different from its actual position.
When viewed from the horizon the position of star appears slightly higher than the actual position.
This apparent position of the star does not remains constant it changes continuously due to changes in the condition of atmosphere.
The star is at large distant apart from our eye which acts as a point source of light. The light coming from star reaching to our eyes gets flicker due to which it appears brighter sometime and fainter sometime.
And this is called as twinkling of stars.
The planets are much closer to the earth’s surface, so they didn’t act as a point source of light.
They are the huge source of light for the observer on the earth.
If we considered planets as a number of point sized sources, then light coming from each source on an average reaching to our eyes becomes zero.
And hence planet do not twinkle like stars..