NCERT Solutions Class 10 Science Chapter 11 The Human Eye and the Colourful World
NCERT Solutions Class 10 Science Chapter 11 The Human Eye and the Colourful World: National Council of Educational Research and Training Class 10 Science Chapter 11 Solutions – The Human Eye and the Colourful World. NCERT Solutions Class 10 Science Chapter 11 PDF Download.
NCERT Solutions Class 10 Science Chapter 11: Overview
Board |
NCERT |
Class |
10 |
Subject |
Science |
Chapter |
11 |
Chapter Name |
The Human Eye and the Colourful World |
Topic |
Exercise Solutions |
NCERT Solutions Class 10 Science Chapter 11 – The Human Eye and the Colourful World
Middle Exercise:
1.) What is mean by the power of accommodation of the eye?
Ans:
- Power of accommodation is the ability of the eye lens to adjust the focal length to see the objects clearly.
- And this change in focal length of the eye lens is caused due to the movement of ciliary muscles.
- When the muscles are relaxed then lens becomes thin and hence focal length of the eye lens increases.
- When the object is very closer to the eye then ciliary muscles contract and eye lens becomes thicker and focal length of eye lens decreases due to which we can see the objects clearly.
- In this way, due to the power of accommodation of eye lens we can see the objects clearly.
2.) 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?
Ans:
- A person with myopic eye cannot see objects beyond 1.2m distinctly because in myopic eye the image of the object which are beyond 1.2m forms in front of the retina and we can’t the see the object clearly.
- The concave lens is used to correct this type of defect of myopia.
3.) What is the far point and near point of the human eye with normal vision?
Ans:
- The minimum distance up to which objects are visible distinctly without any strain on the eye is called as the least distance of the distinct vision or near point of the eye. For normal eye the near point is about 25 cm.
- The point more distant apart up to which the objects are visible is called as the far point of the eye. And for normal eye the far point may be at infinity.
- Hence, the normal eye can see the objects which are in between 25cm and infinity.
4.) 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?
Ans:
Given that, the student has difficulty in reading the blackboard while sitting in the last row that means he doesn’t see the distant objects clearly and distinctly and hence he is suffering from myopia. Myopia can be corrected by using concave lens of proper focal length.
Exercise
1.) The Human eye can focus on object at different distances by adjusting the focal length of the eye lens. This is due to
a) presbyopia
b) accommodation
c) near sightedness
d) far sightedness
Ans:
b) accommodation
Explanation: Because due to power of accommodation of eye human can focus on object at different distances by adjusting the focal length of the eye lens.
2.) The Human eye forms an image of object at its
a) cornea
b) iris
c) pupil
d) retina
Ans:
d) retina
3.) The least distance of distinct vision for a young adult with normal vision is about
a) 25 m
b) 2.5 cm
c) 25 cm
d) 2.5 m
Ans:
c) 25 cm
Explanation: Because, 25cm is the least distance of distinct vision for a young adult with normal vision.
4.) The change in focal length of an eye lens is caused by the action of the
a) pupil
b) retina
c) ciliary muscles
d) iris
Ans:
c) ciliary muscles
Explanation: Because the change in focal length of an eye lens is caused by the action of ciliary muscles.
5.) 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?
a) distant vision
b) near vision
Ans:
Given that,
a) distant vision:
For distant vision the person needs a lens of power -5.5dioptre.
Hence, focal length of the lens required for correcting distant vision is given by,
P= 1/f
Thus, f= 1/P
And f= 1/-5.5 = -0.18 m = -18 cm
b) near vision:
Given that, the person needs a lens of power +1.5 dioptres for correcting his near vision then the focal length of that lens will be,
P=1/f
Thus, f=1/P = 1/+1.5= 0.66m = 66cm
Alternative Answer –
Given that
Power of the lens used for correcting distant vision, P1 = -5.5 D
Power of the lens used for correcting near vision, P2 = +1.5 D
w.k.t power of the lens, P = 1/ f
Where f is the focal length of the lens.
First, we find the focal length of the lens required for correcting distant vision.
From above, P1 = -5.5 D
Therefore required focal length of the lens , f1 = 1/ P1
f1 = 1/ -5.5
f1 = -0.1818 m
Next, we find the focal length of the lens required for correcting near vision
From above, P2 = +1.5 D
Therefore required focal length of the lens, f2 = 1/ P2
f2 = 1 / 1.5
f2 = +0.666 m
Therefore the required focal length of the lens for correcting distant vision and near vision is -0.1818 m and +0.666 m
6.) 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?
Ans:
Given that,
- The far point of a myopic person is 80 cm in front of the eye.
- That means the person is suffering from myopia and for correcting it concave lens of proper focal length should be required.
- Thus, the focal length of the concave lens should be f= -80cm = -0.8m
- And the power of that lens is given by, P= 1/f = 1/-0.8= -1.25D
- Thus, the lens required should be concave lens and the power of lens will be -1.25D.
Alternative Answer –
Any person with myopia. He can only see near objects and cannot see distant object. So he required a such lens which provides clear image of distant object which is Concave lens . Concave lens provides the clear image of distant object for myopia eyes.
w.k.t Power of the lens, P = 1/ f
Where f is focal length of the lens
given that f = -80 cm = -0.8 m
Therefore P = 1/ -0.8
P = -1.25 D
Therefore the required power of the lens to correct the problem is -1.25 D
In case you are missed :- Previous Chapter Solution
7.) Make a diagram to show how hypermetropia is corrected. The near point of the hypermetropic eye is 1m. What is the power of the lens required to correct this defect? Assume the near point of the normal eye is 25cm.
Fig. hypermetropic eye
The following is the diagram showing correction in hypermetropia.
Fig. Correction for Hypermetropic eye using convex lens
We know that, the near point of the normal eye is 25cm.
And given that the person suffering from hypermetropia has near point at 1m.
Hence, we can write
Object distance u= -25cm
And image distance v= -1m = -100cm
By using lens formula,
1/f = 1/v – 1/u
1/f = 1/-100 + 1/25
1/f = 1/25 – 1/100
1/f = (4-1)/100 = 3/100
Thus, f= 100/3 = 33.33 cm = 0.33m
And the power of the lens is given by,
P= 1/f = 1/0.33 = 3D
Thus, the power of the lens will be 3D.
Alternative Answer –
Any person with Hypermetropia . He can see distant object clearly but cannot see nearby objects. So he required a such lens which provides clear image of nearer object which is Convex lens. Convex lens provides the clear image of nearer object for Hypermetropia eyes.
Given that,
The distance of object from an image at the near point of the Hypermetropia eye, u = -25 cm
The near point of the Hypermetropia eye.
Which is image distance, v = 1 m = -100 cm
Using the lens formula, (1/v) – (1/u) = 1/ f
1/ f = (1/-100) – (1/-25)
1/ f = (-25 + 100)/ 2500
= 75 / 2500
= 3 / 100
f = (100 / 3) cm
f = (1 / 3) m
w.k.t Power of the lens, P = 1 /f
P = 1 /( ⅓ )
P = + 3.0 D
Therefore the required power of the lens to correct the diffect is 3D.
8.) Why is normal eye is not able to see clearly the objects placed closer than 25 cm.
Ans:
When the objects are placed near to eye then focal length of the eye lens will be adjusted by contracting ciliary muscles but when the object is kept at a distance less than 25 cm which is the near point of normal eye then the limit of contracting ciliary muscles get exhausted or they becomes unable to contract and hence we cannot see the object clearly which also produces strain on the eyes.
Alternative Answer –
When the distance between the eye and the object is less than 25 cm , the ciliary muscles of the eye need to contract beyond the specific limits of the eye, but that is a task that cannot be done by the ciliary muscles of the eye. In this situation the light rays pass beyond the retina. In this state no object is clearly visible to us.
9.) What happens to the image distance in the eye when we increase the distance of the object from the eye?
Ans:
When the object distance is increased from the eye then there will be no change in the image distance. Because, due to the power of accommodation of eye lens it adjust the focal length of the eye lens and hence the image will be formed on only retina.
Alternative Answer –
When increasing the distance of the object from the eye shows no difference in the distance of the image in the eye. Because as the object moves always from the eye, its lens becomes thinner and the focal length increases,but there is no difference in the distance of the image seen.
10.) Why do star twinkle?
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.
Alternative Answer –
When the starlight enters the Earth’s atmosphere, the starlight is refracted by the Earth’s atmosphere, cousing us to see twinkling stars. As the starlight enters the Earth’s atmosphere, the starlight undergoes a process of refraction to varying degrees due to differences in the density of the air. Because of this refraction at different points, we notice variation in the clarity of the starlight. As the density of air varies, the perspective of a stargazer is also varies.So sometimes we see the Star shinning and sometimes the star appears dim. Thus the stars shines or twinkle.
11.) Explain why planets do not twinkle?
Ans:
- 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.
Alternative Answer –
Planets are closer to Earth compared to bright stars, so planets are not twinkle. And since the planets are not source of light, in the process of refraction coused by the density of air in the Earth’s atmospher, the light of the planets is less affected then that of the star. Thus, the planets do not appear to be twinkling to the eyes of the beholder.
12.) Why does sun appear reddish early in the morning?
Ans:
- The sun appears reddish at the time of sunrise while appears white at noon.
- At the time of 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 sunrise.
Alternative Answer –
Before the sun’s rays reach the Earth in the morning, they enter the Earth’s atmosphere and then enter our eyes. But in this journey, when light passes through the Earth’s atmosphere, not all the colours of the sun’s rays fall into our eyes. As we all know, red colour has a longer wavelength and blue colour has a shorter wavelength. Therefore, only red colour rays pass through the atmosphere and fall into our eyes after scattering of light in the Earth’s atmosphere, so the sun looks reddish in the morning.
13.) Why does sky appear dark instead of blue to an astronaut?
Ans:
In atmosphere on the earth there are large number of dust particles and air molecules which are responsible for scattering of light and hence sky appears blue.
But, in space there is no atmosphere due to which no dust particles and air molecules to scatter the light and hence an astronaut in space see sky as dark.
Alternative Answer –
As we know that the sky appears blue to us, when the rays of the sun enter the Earth’s atmosphere, the light scattering process takes place due to the air,so the sky appears blue to us. But in space there is no air ,so there is no light scattering process,so the sky appears dark to the astronauts in space.
In case you are missed :- Next Chapter Solution