Lakhmir Singh Class 10 Physics 6th Chapter The Human Eye and The Colourful World Solution

Lakhmir Singh Manjit Kaur Physics Solution: The Human Eye and The Colourful World Chapter 6. Here you get easy Solutions of Lakhmir Singh Class 10 Physics Solution Chapter 6. Here we have given Chapter 6 all Solution of Class 10. Its help you to complete your homework.

Board – CBSE
Text Book – Physics
Class – 10
Chapter – 06

Lakhmir Singh Class 10 Physics 6th Chapter Solution

Very Short Answer Type Questions

1) What kind of lens is present in the human eye?

Ans:

Convex lens is present in human eye which is called as eye lens.

2) Name two parts of the eye which refract light rays (or bend light rays).

Ans:

The two parts of the eye which refract light rays or bend light rays are cornea and eye lens.

3) Name the part of the eye:
(a) which controls the amount of light entering the eye.

(b) on which the image is formed)

(c) which changes the focal length of eye-lens)

Ans:

Iris controls the amount of light entering the eye.

b)
In human eye image is formed on the retina.

c)
Ciliary muscles helps in changing the focal length of the eye lens.

4) What is the name of:

(a) the curved, transparent front surface of the eye?

(b) the light-sensitive layer in the eye?

Ans:

a)
The curved, transparent front surface of the eye is called as cornea.

b)
The light sensitive layer in the eye is called as retina.

5) Where is the image formed in a human eye?

Ans:

In human eye image is formed on the retina.

6) What is the function of the lens in the human eye?

Ans:

When light falls on the eye lens which is the convex lens through pupil and it converges the light rays in order to produce real and inverted image of the object on the retina.

7) What job does the pupil of the eye do?

Ans:

The amount of light entering the eye is controlled by changing the size of the pupil.

8) How does the eye adjust to take account of an increase in brightness?

Ans:

When the light of high brightness enters the eye then the iris contracts the pupil due to which it becomes small and hence reduces the amount of light entering the eye) In this way, iris regulates the amount of light entering the eye by changing the size of the pupil.

9) Name that part of the eye which is equivalent to the photographic film in a camera)

Ans:

Retina is the part of human eye which is equivalent to the photographic film in a camera.

10) Name the part of the retina which is insensitive to light.

Ans:

Blind spot is the part of retina which is insensitive to light.

11) Which part of the eye contains cells which are sensitive to light?

Ans:

Retina is the part of eye which contains cells which are sensitive to light.

12) Name two types of cells in the retina of an eye which respond to light)

Ans:

Rods and cones are the two types of cells in the retina of an eye which respond to light.

13) Out of rods and cones in the retina of your eye: (a) which detect colour? (b) which work in dim light?

Ans:

a) Cone cells are responsible for the causing of sensation of colour of objects in our eye.
b)Rod cells are sensitive to dim light.

14) State whether the following statement is true or false: The image formed on our retina is upside-down

Ans:

The above statement is true.

15) What is the principal function of the eye-lens?

Ans:

When light falls on the eye lens which is the convex lens through pupil and it converges the light rays in order to produce real and inverted image of the object on the retina.

16) Where does the greatest degree of refraction of light occur in the eye?

Ans:

At cornea the greatest degree of refraction of light occurs in the eye.

17) What changes the shape of lens in the eye?

Ans:

Ciliary muscles changes the shape of lens in the eye.

18) What do the ciliary muscles do when you are focusing on a nearby object?

Ans:

When we are focusing nearby objects then ciliary muscles makes the eye lens thicker i.e) more converging)

19) What is the least distance of distinct vision for a normal human eye?

Ans:

The least distance of distinct vision for a normal human eye is about 25 centimetres.

20) What is the:

(a) far point of a normal human eye?

(b) near point of a normal human eye?

Ans:

a) Far point of a normal human eye is at infinity.

b) Near point of a normal human eye is about 25cm.

21) What is the range of vision of a normal human eye?

Ans:

The range of the vision of a normal human eye is from infinity to about 25 centimetres.

22) Name the part of our eyes which helps us to focus near and distant objects in quick succession)

Ans:

Ciliary muscles is the part of our eyes which helps us to focus near and distant objects in quick succession.

23) Define the term “power of accommodation” of human eye.

Ans:

Power of accommodation is the ability of eye to focus nearby as well as distant objects on the retina by changing the focal length of the lens)

24. Give the scientific names of the following parts of the eye:
(a) carries signals from an eye to the brain.

(b) muscles which change the shape of the eye-lens.

(c) a hole in the middle of the iris.

(d) a clear window at the front of the eye.

(e) changes shape to focus a picture on the retina.

Ans:

a) Optic nerve carries signals from an eye to the brain

b) Ciliary muscles are the muscles which changes the shape of the eye lens.

c) Pupil is a hole in the middle of the iris.

d) Cornea is the clear window at the front of the eye.

e) Eye lens changes shape to focus a picture on the retina.

25. Fill in the following blanks with suitable words:

Ans:

a) Most of the refraction of light rays entering the eye occurs at the outer surface of the cornea.

b) The part of eye sensitive to light is retina.

c) The part of eye which alters the size of the pupil is iris.

d) When light is dim, the pupil becomes large.

e) The iris controls the amount of light entering the eye.

f) The ciliary muscles controls the shape of the eye lens.

g) To bring light from a distant object to a focus on the retina of the eye, the convex eye lens needs to be made thinner.

h) To bring light from a near object to a focus on the retina of the eye, the convex eye lens needs to be made thicker.

Short Answer Type Questions

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

Ans:

When the objects are very close to the eye than the near point of the eye then we feel lot of strain on the eyes and hence the object looks like blurred.

Hence, we can’t see the objects which are closer to our eye than near point clearly without any strain on the eye.

27. What changes take place in the shape of eye-lens:

(a) when the eye is focused on a near object?

(b) when the eye is focused on a distant object?

Ans:

a) When the eye is focused on a near object then eye lens becomes more thicker i.e. more convex.

b) When the eye is focused on a distant object then eye lens becomes thinner i.e. less convex.

28. The eyes of a person are focused (i) on a nearby object, and (ii) on a distant object, turn by turn. In which case:

(a) the focal length of eye-lens will be the maximum?

(b) the converging power of eye-lens will be the maximum?

Ans:

a) The focal length of eye lens will be maximum when eyes of a person are focused on a distant object.

b) The converging power of eye lens will be maximum when the eyes of a person are focused on a nearby objects.

29. What change is made in the eye to enable it to focus on objects situated at different distances? Illustrate your answer with the help of diagrams.

Ans:

When the eyes are looking at a objects which are more distant apart then the ciliary muscles get totally relaxed which pulls the suspensory ligaments attached to the eye lens tightly.
This causes the lens to stretch and it becomes thin as shown in figure below. This thin eye lens has large focal length and hence less converging power. And this small converging power of lens is enough to converge the parallel rays which are coming from the distant object in order to form the image on the retina.
When eye looks towards the distant objects then it is in the relaxed state and hence the eye is called as unaccommodated.

When our eye is seeing towards the nearby objects then ciliary muscles of our eyes get contracted which makes the suspensory ligaments loose. And due to which the eye lens starts bulging under its own elasticity and get become thick.
This thick eye lens has small focal length and hence large converging power. Due to which it converges the diverging rays coming from the nearby objects in order to form the image on the retina of the eye as shown in figure below.
The condition of eye to become thicker to see nearby objects is said to be accommodated. In this way we can see the nearby objects clearly without any strain on our eye.

30) How is the amount of light entering the eye controlled?

Ans:

The amount of light entering the eye is controlled by the iris of the eye. This can be done by adjusting the size of the pupil according to the intensity of the light incident on our eye.
If the light entering to our eyes is large then iris contracts the pupil and hence reduces the amount of light entering the eye.
While when the amount of light entering the eye is very small the iris expands the pupil and hence more light is enters through our eye.
In this way, the amount of light entering the eye is controlled by iris by adjusting the size of the pupil of the eye.

31. What happens to the eye when you enter a darkened cinema hall from bright sunshine? Give reason for your answer.

Ans:

When we are in the lighter than our eyes control the light entering by contracting the size of the pupil and hence less light enters the eye. But when we enter a darkened cinema hall from bright sunshine then our eyes are not able to see suddenly the objects. But after some time, pupil starts to expand so that the light entering our eyes is more and we see the objects in darkened cinema hall without strain on our eyes.

32) Why does it take some time to see objects in a dim room when you enter the room from bright sunshine outside?

Ans:

When we are in the bright sunshine at that time our eyes receive more light and pupil starts to contract to control the light entering. But when we go from bright sunshine in to a dim room then our eyes are not able to see objects immediately and it takes some time to expand the pupil in order to enter the lighter through our eyes and we can see objects clearly without any strain on our eyes.

33) A person walking in a dark corridor enters into a brightly lit room:

(a) State the effect on the pupil of the eye.

(b) How does this affect the amount of light entering the eye?

Ans:

a) When a person walking in a dark corridor enters into a brightly lit room then the pupil becomes smaller.
b) As the size of pupil becomes smaller the amount of light entering the eye get reduced.

34. Ciliary muscles of human eye can contract or relax. How does it help in the normal functioning of the eye?

Ans:

The ciliary muscles help in contracting or relaxing the eye lens and due to which the focal length of the eye lens changes which helps in seeing the objects which are at nearby distance as well as more distant apart front the eye.

When the eyes are looking at a object which are more distant apart then the ciliary muscles get totally relaxed which pulls the suspensory ligaments attached to the eye lens tightly.
This causes the lens to stretch and it becomes thin as shown in figure below. This thin eye lens has large focal length and hence less converging power. And this small converging power of lens is enough to converge the parallel rays which are coming from the distant object in order to form the image on the retina.
When eye looks towards the distant objects then it is in the relaxed state and hence the eye is called as unaccommodated.

When our eye is seeing towards the nearby objects then ciliary muscles of our eyes get contracted which makes the suspensory ligaments loose. And due to which the eye lens starts bulging under its own elasticity and get become thick.
This thick eye lens has small focal length and hence large converging power. Due to which it converges the diverging rays coming from the nearby objects in order to form the image on the retina of the eye as shown in figure below.
The condition of eye to become thicker to see nearby objects is said to be accommodated. In this way we can see the nearby objects clearly without any strain on our eye.

35. Describe and explain, how a normal eye can see objects lying at various distances clearly.

Ans:

We can see the objects which are at nearby or more distant apart due to the power of accommodation of eye. It is the ability of eye to see the objects which are nearby and more distant from the eye without any strain on our eye.
The ciliary muscles help in contracting or relaxing the eye lens and due to which the focal length of the eye lens changes which helps in seeing the objects which are at nearby distance as well as more distant apart front the eye.

When the eyes are looking at a object which are more distant apart then the ciliary muscles get totally relaxed which pulls the suspensory ligaments attached to the eye lens tightly.
This causes the lens to stretch and it becomes thin as shown in figure below. This thin eye lens has large focal length and hence less converging power. And this small converging power of lens is enough to converge the parallel rays which are coming from the distant object in order to form the image on the retina.
When eye looks towards the distant objects then it is in the relaxed state and hence the eye is called as unaccommodated.

When our eye is seeing towards the nearby objects then ciliary muscles of our eyes get contracted which makes the suspensory ligaments loose. And due to which the eye lens starts bulging under its own elasticity and get become thick.
This thick eye lens has small focal length and hence large converging power. Due to which it converge the diverging rays coming from the nearby objects in order to form the image on the retina of the eye as shown in figure below.
The condition of eye to become more thick to see nearby objects is said to be accommodated. In this way we can see the nearby objects clearly without any strain on our eye.

36) There are two types of light-sensitive cells in the human eye:

(a) Where are they found?

(b) What is each type called?

(c) To what is each type of cell sensitive?

Ans:

a) There are two types of light sensitive cells in human eye which are found in the retina of our eye.

b) There are two types of light sensitive cells present in the retina of our eye they are namely rods and cones.

c) Rods cells are sensitive to dim light. And cones cells are sensitive to bright light and also cone cells cause the sensation of colour of objects in our eyes.

37) What are rods and cones in the retina of an eye? Why is our night vision relatively poor compared to the night vision of an owl?

Ans:

There are two types of light sensitive cells present in the retina of our eye they are namely rods and cones.
Rods cells are sensitive to dim light. And cones cells are sensitive to bright light and also cone cells cause the sensation of colour of objects in our eyes.
The nocturnal animals like owl has large number of rod cells in their retina due to which they can see the objects even in the less light.
Our night vision is relatively poor as compared to night vision of an owl because in the retina of human eye the rod cells are present in smaller amount.

38. (a) How does the convex eye-lens differ from the ordinary convex lens made of glass?

(b) List, in order, the parts of the eye through which light passes to reach the retina.

Ans:

a) In human eye, the focal length of convex lens i.e. eye lens is adjustable by the contraction and relaxation of ciliary muscles. But in case of convex lens made of glass the focal length of that lens is fixed we can’t change it or adjust it.

b) Following is the order of the parts of the eye through which light passes to reach the retina.

Cornea –> pupil —> eye lens –> retina

39. (a) What happens to the size of pupil of our eye (i) in dim light (ii) in bright light?

(b) Name the cells on the retina of an eye which are sensitive to (i) bright light (ii) dim light (iii) sensation of colour.

Ans:

a) 1) in dim light the size of the pupil becomes large.

2) in bright light the size of the pupil becomes small.

1) cones are the cells on the retina of an eye which are sensitive to bright light.

2) rods are the cells on the retina of an eye which are sensitive to dim light.

3) cones are the cells on the retina of an eye which are sensitive to sensation of colour.

Long Answer Type Questions

40. (a) Draw a simple diagram of the human eye and label clearly the cornea, iris, pupil, ciliary muscles, eye lens, retina, optic nerve and blind spot.

(b) Describe the working of the human eye with the help of the above diagram.

(c) How does the eye adjust itself to deal with light of varying intensity?

Ans:

a) The following diagram shows the human eye with cornea, iris, pupil, ciliary muscles, eye lens, retina, optic nerve and blind spot.

The working of the human is as explained follows:
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.

The amount of light entering the eye is controlled by the iris of the eye. This can be done by adjusting the size of the pupil according to the intensity of the light incident on our eye.
If the light entering to our eyes is large then iris contracts the pupil and hence reduces the amount of light entering the eye.
While when the amount of light entering the eye is very small the iris expands the pupil and hence more light is enters through our eye.
In this way, the amount of light entering the eye is controlled by iris by adjusting the size of the pupil of the eye.

41. (a) Explain the functions of the following parts of the eye:

(a) cornea

(b) iris

(c) pupil

(d) ciliary muscles

(e) eye-lens

(f) retina

(g) optic nerve

(b) If you walk from a dark room into sunlight and back again into dark room, how would your pupils alter in size? What makes this happen?

(c) Explain why, we cannot see our seats first when we enter a darkened cinema hall from bright light but gradually, they become visible.

Ans:

1) The light rays coming from the objects enters the eye through cornea.

2) Iris controls the amount of light entering the eye.

3) Pupil contracts or relaxes according to the intensity of light and regulates the light entering the eye.

4) by the action of the ciliary muscles the focal length of the convex lens i.e. eye lens can be changed or adjusted.

5) eye lens changes its shape in order to focus the light on to the retina.

6) on retina of the human the image is formed.

7) optic nerve helps in transferring visual information from retina through electrical impulse to the vision centres of the brain.

When we are in the dark room then the pupil expands in order to pass more light through it. And we can see objects clearly even in dark also. But when we enter from dark into sunlight our eyes feel like glare because pupil is expanded then after sometime again pupil starts contracting in order to reach small amount of light through it.
And we can see the objects in sunlight clearly without any strain on our eye.
But, when again we go into dark room at that time our pupil is contracted due to which we can’t see the objects clearly in dark but after sometime our pupil starts expanding again so that large amount of light reach to it. And we see objects in dark clearly without any strain on our eye.
Thus, iris controls the amount of light entering the eye by changing or adjusting the size of the pupil according to the intensity of light incident on it.

When we enter darkened cinema hall from bright light we can’t see the seats clearly because at that time our pupil becomes contracted due to which less amount of light enters the eye and hence we can’t see the seats in dark.
But after sometime the pupil starts expanding due to which large amount of light enters the eye due to which we can see the seats in darkened cinema hall.

DEFECTS OF VISION AND THEIR CORRECTION

Very Short Answer Type Questions

1. Name one of the common defects of vision and the type of lens used to remove it.
Ans:

Myopia is the common defect of vision in which a person can see the nearby objects clearly and distinctly but can’t see objects which are more distant apart from the eye.
This defect is corrected by using concave lens of proper focal length.

2. Name the defect of vision in a person:

(a) who’s near point is more than 25 cm away.

(b) whose far point is less than infinity

Ans:

a)
In hypermetropia the near point is more than 25cm away.

b)
In myopia the far point is less than infinity.

3. Which defect of vision can be rectified:
(a) by using a concave lens?

(b) by using a convex lens?

Ans:

a) Myopia is the defect of vision which is rectified by using a concave lens.

b) Hypermetropia is the defect of vision which is rectified by using a convex lens.

4. What type of lens is used to correct (a) hypermetropia (b) myopia?

Ans:

a)to correct hypermetropia convex lenses of proper focal length are used.

b)To correct myopia concave lenses of proper focal length are used.

5. What is the other name for (a) myopia (b) hypermetropia?

Ans:

a)The other name of myopia is near-sightedness.

b)The other name of hypermetropia is far sightedness.

6. What is the scientific name of (a) short-sightedness, and (b) long-sightedness?

Ans:

a)The scientific name of short sightedness is myopia.

b)The scientific name of the long sightedness is hypermetropia.

7. What kind of lens is used to correct (a) short-sightedness (b) long-sightedness?

Ans:

a) Concave lens is used to correct the short sightedness.

b) Convex lens is used to correct the long sightedness.

8. State whether the following statement is true or false:

1) Short-sightedness can be cured by using a concave lens.

Ans:

The above statement is true.

9. Name the defect of vision in which the eye-lens loses its power of accommodation due to old age.

Ans:

Presbyopia is the defect of vision in which the eye lens loses its power of accommodation due to old age.

10. Name the defect of vision which makes the eye-lens cloudy resulting in blurred vision.

Ans:

Cataract is the defect of vision which makes the eye lens cloudy resulting in blurred vision.

11. What is the other name of old age hypermetropia?

Ans:

The other name of old age hypermetropia is presbyopia.

12. Name any two defects of vision which can be corrected by using spectacles.

Ans:

The following are the two defects of vision which can be corrected by using spectacles.
Myopia and hypermetropia.

13. Name one defect of vision (or eye) which cannot be corrected by any type of spectacle lenses.

Ans:

Cataract is one of the defects of vision which cannot be corrected by any type of spectacle lenses.

14. Name the body part with which the terms myopia and hypermetropia are connected.
Ans:

Eye is the body part with which the terms myopia and hypermetropia are connected.

15. What is the far point of a person suffering from myopia (or short-sightedness)?
Ans:

The far point of a person suffering from myopia or short sightedness is located closer than the infinity.

16. Where is the near point of a person suffering from hypermetropia (or long-sightedness) ?

Ans:

The near point of a person suffering from hypermetropia or long sightedness is more than 25cm away.

17. Your friend can read a book perfectly well but cannot read the writing on blackboard unless she sits on the front row in class.

(a) Is she short-sighted or long-sighted?

(b) What type of lenses–converging or diverging–would an optician prescribe for her?

Ans:

a) She is short sighted.

b) Diverging lenses will be preferred for her by an optician.

18. A man can read the number of a distant bus clearly but he finds difficulty in reading a book.

(a) From which defect of the eye is he suffering?

(b) What type of spectacle lens should he use to correct the defect?

Ans:

a)He is suffering from the defect hypermetropia or long sightedness.
b)This type defect can be corrected by using convex lenses of proper focal length.

19. A student sitting in the last row of the class-room is not able to read clearly the writing on the blackboard.

(a) Name the type of defect he is suffering from.

(b) How can this defect by corrected?

Ans:

a)
The student is suffering from the defect called as myopia or short sightedness.
b)
This defect can be corrected by using concave lens of proper focal length.

20. Complete the following sentences:

Ans:

a)A short-sighted person cannot see distant objects clearly. Short-sightedness can be corrected by using concave lenses.
b)A long-sighted person cannot see nearby objects clearly. Long sightedness can be corrected by using convex lenses.

Short Answer Type Questions

21. What are the two most common defects of vision (or defects of eye)? How are they corrected?

Ans:

There are two most common defects of vision they are namely myopia and hypermetropia.
Myopia is corrected by using concave lens of proper focal length while hypermetropia is corrected using convex lens of proper focal length.

22. Differentiate between myopia and hypermetropia. What type of spectacles should be worn by a person having the defects of myopia as well as hypermetropia? How does it help?

Ans:

Myopia:

Myopia is also called as short sightedness or near sightedness.
The person suffering from myopia can see the nearby objects clearly and distinctly but can’t see objects which are more distant apart from the eye.
The far point of myopic eye is less than infinity.
Myopia is causes due to high converging power of the lens.
Myopia can be corrected by using spectacles containing concave lenses.
Due to concave lenses used the converging power of lenses get reduced and helps in forming the image of the distant objects on the retina of the myopic eye.

Hypermetropia:

Hypermetropia is also called as long sightedness or far sightedness.
The person suffering from hypermetropia can see the objects which are more distant apart from the eye but can’t see the objects which are near to our eyes clearly and distinctly.
The near point of the hypermetropic eye is more than 25cm away.
Hypermetropia causes due to the low converging power of the lens.
Hypermetropia can be corrected by using spectacles containing convex lenses.
Due to convex lenses used the converging power of lens increases and also helps in forming the image of a nearby object on the retina of the eye.

23. Name the defect of vision which can be corrected by a converging lens. Show clearly by a ray diagram how the lens corrects the defect.

Ans:

Hypermetropia is the defect of vision which can be corrected by using a converging lens or convex lens.
The following diagram shows how convex lens is used to correct the hypermetropia.

24) Name the defect of vision which can be corrected by a diverging lens. Show clearly by a ray diagram how the lens corrects the defect.

Ans:

Myopia is the defect of vision which can be corrected by using a diverging lens i.e. concave lens.

The following diagram shows how concave lens is used to correct the myopia.

25) Explain with the help of labelled ray diagram, the defect of vision called myopia and how it is corrected by a lens.

Ans:

Myopia is also called as short sightedness or near sightedness.
The person suffering from myopia can see the nearby objects clearly and distinctly but can’t see objects which are more distant apart from the eye.
The far point of myopic eye is less than infinity.
Myopia is causes due to high converging power of the lens.
Myopia can be corrected by using spectacles containing concave lenses.
Due to concave lenses used the converging power of lenses get

26) Explain with the help of labelled ray-diagram, the defect of vision called hypermetropia, and how it is corrected by a lens.

Ans:

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.
This defect can be corrected as shown in fig 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. Hypermetropic eye

Fig. Correction for hypermetropic eye using convex lens

27) A person suffering from the eye-defect myopia (short-sightedness) can see clearly only up to a distance of 2 metres. What is the nature and power of lens required to rectify this defect?

Ans:

We know that, the person suffering from myopia can see nearby objects clearly and distinctly but can’t see the objects which are more distant apart from the eye. To correct such defect concave lens of proper focal length is used.

Here, a person suffering from eye defect myopia can see clearly only up to a distance of 2 meters.

Hence, image distance v = -2m

Object distance u = infinity

We know that, 1/f = 1/v – 1/u

Thus, 1/f = -1/2 – 0 = -1/2

Thus, f = -2m

Now, the power of lens is given by,

P= 1/f = 1/-2= -0.5D

Hence, here the defect myopia can be corrected by using concave lens of power -0.5D.

28) The near-point of a person suffering from hypermetropia is at 50 cm from his eye. What is the nature and power of the lens needed to correct this defect? (Assume that the near-point of the normal eye is 25 cm).

Ans:

We know that, the person suffering from hypermetropia can see objects which are more distant apart from the eye but can’t see the objects which are nearby. This defect can be corrected by using convex lens of proper focal length.

Given that, the near point of the person suffering from hypermetropia is at 50cm from his eye.

Hence, here image distance v = -50cm

Object distance u= -25cm

We know that,

1/f = 1/v – 1/u = -1/50 + 1/25 = 1/50

Thus, f = 50cm = 0.5m

Now, the power of the lens is given by,

P= 1/f = 1/0.5= 10/5= 2D

Thus, here to correct the hypermetropia convex lens of power 2D is needed.

29) 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 dioptres. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision?

Ans:

For distant vision:

Given that, power of lens = -5.5D

We know that,

P= 1/f hence, f = 1/P = 1/-5.5 = -10/55= -2/11= -0.181m= -18.18cm

Thus, the distant vision can be corrected by using lens of focal length -18.18cm

For near vision:

Given that, power of lens = +1.5D

We know that, P = 1/f

Thus, f = 1/P = 1/1.5= 0.667m = 66.7cm

Thus, the near vision can be corrected by using lens of focal length 66.7cm

30) What is presbyopia? Write two causes of this defect. Name the type of lens which can be used to correct presbyopia.

Ans:

Presbyopia is the defect of vision which mostly found in old person who cannot see the nearby objects clearly and distinctly due to loss of power of accommodation of the eye.
The presbyopia causes due to weaken of ciliary muscles due to which eye lens get become rigid. And also due to loss of power of accommodation of eye.
Presbyopia can be corrected by using convex lenses of proper focal length.

31) When is a person said to have developed cataract in his eye? How is the vision of a person having cataract restored?

Ans:

The eye lens of person suffering from cataract becomes cloudy which results in blurred vision.
Cataract occurs when the eye lens becomes cloudy due to the formation of membrane over it. The person suffering from cataract loses vision gradually and finally it may lead to total loss of vision also.
By doing surgery the vision of a person having cataract restored.

32) Fill in the following blanks with suitable words:

Ans:

A person is short sighted if his eyeball is too long. Spectacles with a concave lens are needed.
A person is long sighted if his eyeball is too short. Spectacles with a convex lens are needed.
These focus light rays exactly on to the retina.

Long Answer Type Questions

33) (a) What is short-sightedness? State the two causes of short-sightedness (or myopia). With the help of ray diagrams, show:

(i) the eye-defect short-sightedness.

(ii) correction of short-sightedness by using a lens.

(b) A person having short-sight cannot see objects clearly beyond a distance of 1.5 m. What would be the nature and power of the corrective lens to restore proper vision?

Ans:

Myopia is also called as short sightedness. In which a person can see nearby objects clearly and distinctly but can’t see objects which are more distant apart from the eye.

The myopia causes due to high converging power of the eye lens or due to eye ball becomes too long.

The following diagram shows the eye defect short sightedness or myopia.

The following diagram shows the correction of short sightedness by using a concave lens.

We know that, the defect myopia can be corrected by using the concave lens of proper focal length. Here, the person having myopia cannot see objects clearly beyond a distance of 1.5m.

Hence, image distance v = – 1.5m

And object distance u = infinity

We know that,

1/f = 1/v – 1/u = -1/1.5 – 0 = -1/1.5= -10/15 = -2/3

Thus, f = -3/2= -1.5m

The power of the lens is given by, P = 1/f = -1/1.5= -0.67D

Here, to correct the defect myopia concave lens of power -0.67D is used.

34) (a) What is long-sightedness? State the two causes of long-sightedness (or hypermetropia). With the help of ray diagrams, show:

(i) the eye-defect long-sightedness.

(ii) correction of long-sightedness by using a lens.

(b) An eye has a near point distance of 0.75 m. What sort of lens in spectacles would be needed to reduce the near point distance to 0.25 m? Also calculate the power of lens required. Is this eye long-sighted or shortsighted?

(c) An eye has a far point of 2 m. What type of lens in spectacles would be needed to increase the far point to infinity? Also calculate the power of lens required. Is this eye long-sighted or short-sighted?

Ans:

Hypermetropia is also called as long sightedness or far sightedness. The person suffering from this defect of vision can see objects which are more distant apart from the eye but cannot see the objects which are nearby clearly and distinctly.

Hypermetropia is caused due to low converging power of the eye lens or due to shortening of eye balls.

1) The following diagram shows the eye defect long sightedness.

Fig. Hypermetropic eye

The following diagram shows the correction of long sightedness by using convex lens of proper focal length.

Fig. Correction for Hypermetropic eye using convex lens

We know that, the person suffering from hypermetropia can be corrected by using convex lens of proper focal length.

Here, given that,

Object distance u = -0.25m

Image distance v = – 0.75m

The focal length of the lens is given by,

1/f = 1/v – 1/u = -1/0.75 + 1/0.25 = -1.33 + 4 = 2.67m

Thus, f = 1/(2.67) = 0.3745m

Thus, the power of the lens is given by

P=1/f= 1/0.3745= 2.67D

Thus, here the defect hypermetropia can be corrected by using convex lens of power 2.67D.

The far point of the lens can be increased by using concave lens of proper focal length.

Given that,

Object distance u= infinity

Image distance= v = -2m

We know,

1/f = 1/v -1/u = -1/2 – 0 = -1/2

Thus, f = -2m

The power of the lens is given by,

P= 1/f = -1/2= -0.5D

Thus, concave lens of power -0.5D is needed to correct the above defect of vision.

Hence, the eye is suffering from short sightedness.

WHY DO WE HAVE TWO EYES FOR VISION AND NOT JUST ONE

Very Short Answer Type Questions

(1) Solution:

Ans:

(a) Our field of view with one eye open is about 150°.

(b) Our field of view with both eyes open is about 180°.

(2) Solution:

Ans:

Animals having two eyes on the opposite sides of their head have a wilder field of view.

(3) Solution:

Ans:

(1) Predators have their eyes at the front of their head.

(2) Animals of prey have their eyes on the opposite sides of their head.

(4) Solution:

Ans:

Rabbit has eyes which look sideways. This statement is true.

(5) Solution:

Ans:

(a) Having two eyes enables us to judge wider field of view.

(b) Having two eyes enables us to judge distances more accurately.

Short Answer type Questions

(6) Solution:

Ans:

Following are the two advantages of having two eyes instead of just one:

Having two eyes gives a wider field of view than the single one.
Having two eyes enables us to judge distances more accurately than the single one.

(7) Solution:

Ans:

Because, having two eyes gives us a wider field of view. With one eye open a human being has a horizontal field of view as 150°. But with two eyes open the horizontal field of view becomes 180°.
And hence, having two eyes is more advantageous than having only one eye.
Two eyes give larger field of view than with only one eye. Hence, we can see larger field with two eyes in front of us.
Also, having two eyes enables us to judge more distance accurately as compared to the having a single eye.
Because of the above all reasons, a person who has lost sight of one eye is at a disadvantage compared with the normal person who has two good eyes.

(8) Solution:

Ans:

(a) Rabbit and deer are having eyes on the sides of the head.

(b) Tiger and lion are the two animals having eyes at the front of the head.

(9) Solution:

Ans:

Among the animals, the predators like lions have their eyes facing forward at the front of their heads due to which they can judge the distance of their prey accurately and hence catch them easily.
While the animals of prey like rabbit usually have eyes at the sides of their head due to which they can see their enemies in a very large area of view around them and hence they can be easily escaped from them.

(10) Solution:

Ans:

(a) The person A and C having diabetes and asthma respectively can donate eyes.

(b) The person’s B, D and E having leukaemia, meningitis and hepatitis respectively cannot donate eyes.

GLASS PRISM

Very Short Answer Type Questions

(1) Solution:

Ans:

As light rays pass from air into a glass prism, then the rays are refracted towards the normal.

Since, when light rays go from rarer medium to denser medium it bends towards the normal.

(2) Solution:

Ans:

As light rays emerge from a glass prism into air, then light rays get refracted away from the normal. Since, when light rays travel from denser medium to rarer medium it bends away from the normal.

(3) Solution:

Ans:

The natural phenomena which is caused by the dispersion of sunlight in the sky is rainbow.

(4) Solution:

Ans:

Sunlight consists of 7 colours this information we get about the sunlight from the formation of rainbow.

(5) Solution:

Ans:

Newton demonstrates the phenomenon of dispersion of light by his experiment with the prism. In this phenomenon of dispersion of light, he proved that the sunlight is made from seven colours or sunlight consists of seven colours and they are VIBGYOR i.e., violet, indigo, blue, green, yellow, orange and red.

(6) Solution:

Ans:

The seven colours make up white light and they are VIBGYOR i.e. violet, indigo, blue, green, yellow, orange and red.

(7) Solution:

Ans:

The meaning of the term VIBGYOR is that each letter in it represents the colour and they are namely Violet, Indigo, Blue, Green, Yellow, Orange and Red.

And they are connected with the phenomenon of dispersion of light.

(8) Solution:

Ans:

(a) Red colour is deviated least in the formation of spectrum of white light by a prism.

(b) Violet colour is deviated most in the formation of spectrum of white light by a prism.

(9) Solution:

Ans:

Yellow and blue are the colours lie on the two sides of the green colour in the spectrum of white light.

(10) Solution:

Ans:

The scientist Sir Isaac Newton discovered that sunlight consists of seven colours.

(11) Solution:

Ans:

The order of colours in a rainbow from the outside to inside is as follows:

Red, Orange, Yellow, Green, Blue, Indigo and Violet.

(12) Solution:

Ans:

(a) Red colour of the spectrum has longest wavelength.

(b) Violet colour of the spectrum has shortest wavelength.

(13) Solution:

Ans:

Red light has the longer wavelength.

(14) Solution:

Ans:

Violet colour has the shortest wavelength.

(15) Solution:

Ans:

(a) When a ray of light enters a prism, it bends towards the normal; as it leaves the prism, it bends away from the normal.

(b) White light is composed of seven colours. The colour of white light deviated through the largest angle by a prism is violet.

Short Answer Type Questions

(16) Solution:

Ans;

(1) The following diagram shows the when a ray of light is passed through glass prism it spreads into its components as VIBGYOR.

Following fig. shows the dispersion through a prism when a narrow beam of light is incident on one of its refracting surfaces and the order of the colours obtained from bottom to top is VIBGYOR

(2) The diagram shows the order of colours deviated also.

(3) The different colours of light have different wavelengths and hence they have different speed also. Due to which they refract differently when passed through the prism and hence has different deviations.

(17) Solution:

Ans:

(a) When a ray of ordinary light us passed through the triangular glass prism then prism splits that light into its seven colours.

This spreading of white light into its constituent colours is called as dispersion of light.

(b) If another similar glass prism is placed upside down behind the first prism, then the seven colours emitted from the first prism get reunited again and a ordinary white light is emerged out from the second prism which is placed upside down behind the first prism.

And this is happened only due to the equal and opposite refraction between these two prisms.

(18) Solution:

Ans:

When a beam of white light is passed through the prism, it gets splitter into its seven colours. Again, we can recombine these seven colours of light by placing the second prism upside down behind the first prism as shown in figure below.
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 colours.
Again, he passed this spectrum of light through second identical prism to split this seven colours 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 concludes that sunlight is made up from seven colours which are VIBGYOR.

(19) Solution:

Ans:

(a) When a white light is passed through the glass prism then we get the band of seven colours on the screen which is called as spectrum of light.

And the glass shape used to produce a spectrum is the glass prism.

(b) There are seven colours in a full spectrum of white light. The order of colours in the spectrum of light is Red, Orange, Yellow, Green, Blue, Indigo, Violet.

(20) Solution:

Ans:

The phenomenon of splitting of white light into its seven colours when it is passed through the glass prism is called as dispersion of light.

The following diagram shows the formation of rainbow.

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 raindrop at right angle then it splits into spectrum of seven colours which we called as rainbow.
The rainbow is always formed at opposite side of the sun.

(21) Solution:

Ans:

a) The colour X is the Violet and the colour Y is the Red.

b) The different colours of light have different wavelengths and hence different speed through the prism. Due to which for each colour of light the refractive index is different for prism. Due to which they get deviated with different angles of deviations with respect to the incident beam of light.

(22) Solution:

Ans:

The following diagram shows the dispersion of white light into its constituent colours when passed through the glass prism.

Following fig. shows the dispersion through a prism when a narrow beam of light is incident on one of its refracting surfaces and the order of the colours obtained from bottom to top is VIBGYOR

(23) Solution:

Ans:

The following diagram explains the phenomenon of refraction of light.

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 following diagram explains the phenomenon of dispersion of light.

Following fig. shows the dispersion through a prism when a narrow beam of light is incident on one of its refracting surfaces and the order of the colours obtained from bottom to top is VIBGYOR

(24) Solution:

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 colours.
Again, he passed this spectrum of light through second identical prism to split these seven colours 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 concludes that sunlight is made up from seven colours which are VIBGYOR.

(25) Solution:

Ans:

The following diagram explains the colours of spectrum combine to give white light again.

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 colours.
Again, he passed this spectrum of light through second identical prism to split these seven colours 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 concludes that sunlight is made up from seven colours which are VIBGYOR.

(26) Solution:

Ans:

The violet colour is refracted most by the prism because violet colour has the minimum speed in the glass prism.

Long Answer Type Question

(27) Solution:

Ans:

(a)

The following diagram shows the refraction of the light through a glass prism.

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

(b)

rainbow is the spectrum of seven colours formed in the sky when sunlight get dispersed through the water droplets after rain.

The two main conditions for the formation of rainbow are at that time sun should be shining and also there should be raining.

(c)

Small raindrops act as the tiny prism in the formation of a rainbow.

(d)

The process involved in the formation of rainbow is dispersion of sunlight.

(e)

The seven colours seen in the rainbow are VIBGYOR i.e., violet, indigo, blue, green, yellow, orange and red.

ATMOSPHERIC REFRACTION

Very Short Answer Type Questions

(1) Solution:

Ans:

The phenomenon of atmospheric refraction of light causes the twinkling of stars.

(2) Solution:

Ans:

The following are the two effects produced by the atmospheric refraction:

(1) Twinkling of stars

(2) Advance sunrise and delayed sunset

(3)

(3) Solution:

Ans:

a) The phenomenon of atmospheric refraction of sunlight makes us see the sun a few minutes before actual sunrise.
b) The phenomenon of atmospheric refraction of sunlight makes us see the sun a few minutes after the actual sunset.

(4) Solution:

Ans:

(a) Sunrise advanced about 2 minutes

(b) Sunset delayed about 2 minutes

(5) Solution:

The planets twinkle at night due to atmospheric refraction of light.

Ans:

The above statement is false.

Because planets do not twinkle.

(6) Solution:

Ans:

The phenomenon of atmospheric refraction of sunlight causes the stars seem higher in the sky than they actually are.

(7) Solution:

Ans:

We can see the sun about two minutes before the actual sunrise and about two minutes after the actual sunset because of atmospheric refraction of sunlight.

Short Answer Type Questions

(8) Solution:

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 remain 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 following fig. explains the twinkling of stars in detail.

(9) Solution:

Ans:

We can see the sun about two minutes before actual sunrise due to the atmospheric refraction of light.
When the sun is above the horizon at that time actual sunrise takes place. But, due to atmospheric refraction of light we see the sun just two minutes before the sunrise above the horizon.
This is because, when sun is below the horizon the light rays get refracted from rarer medium to denser medium towards the normal. Due to which we see the sun just above the horizon when it is in actual below the horizon.
And this is the main reason due to which sun can be seen about two minutes before the actual sunrise as explained in the following figure.

(10) Solution:

Ans:

If we look at objects through the hot air over a fire the objects appear to be moving or shaking slightly because of the refraction of light. The air near to fire get heated die to which it become more rarer and the air above it is cold which became denser.

Thus, the light coming from objects travels through air of different optical density and hence undergoes refraction and due to which we see that objects appear to be moving or shaking slightly.

Long Answer Type Question

(11) Solution:

Ans:

(a)

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.

(b)

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 remain 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.

(c)

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.

SCATTERING OF LIGHT

(1) Solution:

Ans:

(a) The light scattered by the dust particles in the atmosphere is white in colour.

(b) The light scattered by the air molecules in the atmosphere is blue in colour.

(2) Solution:

Ans:

Light if shorter wavelength scatters more easily than the light of longer wavelength.

(3) Solution:

(a) The scattering away of red light makes the sky appear blue during the day time.

(b) The scattering away of blue light makes the sun appear red at sunset.

Ans:

(a) False

(b) True

(4) Solution:

Ans:

The sky appears to an astronaut is dark or black in colour.

(5) Solution:

Ans:

When a beam of sunlight enters a dusty room, then it’s path becomes visible to us this effect is illustrated by Tyndall effect i.e. scattering of light.

(6) Solution:

Ans:

Following are the two effects produced by the scattering of light by the atmosphere:

Sky appears blue
Sun appears red at sunrise and sunset.

Short Answer Type Questions

(7) Solution:

Ans:

When light get scattered due to particles in its path is called as Tyndall effect.
When we see the beam of light which is entering in the dusty room through a window or some small hole then the path of the beam is visible to us.
This path of beam is visible because when light enters the dusty room it get scattered from the dust particles present in the air. And when this scattered light enters our eyes we see that bean of light as visible path.
In this way, due to Tyndall effect i.e. scattering of light from dust particles makes path of beam of light visible to us.

(8) Solution:

Ans:

When light get scattered due to particles in its path is called as Tyndall effect.
When we see the beam of light which is entering in the dusty room through a window or some small hole then the path of the beam is visible to us.
This path of beam is visible because when light enters the dusty room it gets scattered from the dust particles present in the air. And when this scattered light enters our eyes, we see that bean of light as visible path.
In this way, due to Tyndall effect i.e. scattering of light from dust particles makes path of beam of light visible to us.

(9) Solution:

Ans:

The blue colour 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 colours.
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 colours are scattered more in atmosphere but violet light is not more sensitive to our eyes and we see blue colour mostly, so the sky appears blue.

(10) Solution:

Ans:

The sky appears dark or black to an astronaut instead of blue because there in no atmosphere which contains air in the outer space. And due to which no scattering of light occurs and hence no scattered light will reach to our eyes. Thus, we see the sky as dark or black. This is the reason due to which an astronaut sees the sky as dark or black instead of blue.

(11) Solution:

Ans:

The sun appears reddish at the time of 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 sunrise.
• The diagram below explains the above discussed topic.

(12) Solution:

Ans:

The sun appears reddish at the time of sunset 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.
The diagram below explains the above discussed topic.

(13) Solution:

Ans:

Red colour has longest wavelength than all the other colours. And also,it gets scattered least due to fig, smoke and dust particles in the air due to which we can see red colour from long distance also.

Because if this, the danger signal lights are made red in colour.

Long Answer Type Question

14.) Solution:

Ans:

(a) The following is the neat and labelled diagram of the experimental set up for observing the scattering of light in a colloidal solution of sulphur to show how the sky appears blue, and the sun appears red at sunrise and sunset.

(b) Out of blue light and red light, blue light is scattered more easily.

(d) Gas molecules present in the air causes the scattering of blue component of sunlight in the atmosphere.

Out of blue light and red light, blue light is scattered more easily.

Blue component of sunlight is scattered away when the sun appears red at sunrise or sunset.

Gas molecules present in the air causes the scattering of blue component of sunlight in the atmosphere.

For more Chapter solution, click below