## Solutions of The Human Eye and The Colourful World Lakhmir Singh Manjit Kaur LAQ, MCQ, VSAQ and SAQ Pg No. 271 Class 10 Physics

Long Answer Type Questions-Pg-271

40A. 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.

40B. Describe the working of the human eye with the help of the above diagram.

→ The working of the human eye starts with the light rays coming from the object kept in front of the eye enter the cornea, pass through the pupil and fall on the eye lens. The eyes lens is convex lens, so it converges the light rays and produces a real and inverted image of the object on the retina. The image formed on the retina is conveyed to the brain by the optic nerve and gives rise to the sensation of vision.

40C. How does the eye adjust itself to deal with light of varying intensity?

→ To deal with light of varying intensity the eye adjust itself by the help of the iris. The iris adjust the pupil automatically according to the intensity of light received by the eye. If the intensity of light is large, then iris contracts the pupil and reduces the amount of light entering the eye. And, if the intensity of light is small, then iris expands the pupil so that more light may enter the eyes.

41A. 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

→ (a) The function of the cornea is to allow the light to enter the eye. It is the front part of the eye.
(b) The function of the iris is to control the amount of light entering the eye.
(c) The function of the pupil is to control the illumination in the eye.
(d) The function of the ciliary muscles is to change the focal length of the eye-lens by changing its shape.
(e) The function of the eye-lens is to focuses light on to the retina.
(f) The function of the retina is respond to the intensity of light and colour of objects respectively with the help of rods and cones present in it.
(g) The function of the optic nerve is to convey the image formed on the retina to the brain.

41B. 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?

→ The pupil of the eye contracts, if we walk from a dark room into sunlight. But it expands on entering in the dark room. This happens because of the intensity of the light changes.

41C. Explain why, we cannot see our seats first when we enter a darkened cinema hall from bright light but gradually they become visible.

→ We cannot see our seats first when we enter a darkened cinema hall from bright light but gradually they become visible. This is due to the reason because in bright sunshine the pupil of our eye is small and when we just enter the darkened room very little light enters our eye due to which we cannot see properly. After a while, when the pupil of our eye expands, more light enters our eye and we can see clearly.

Multiple Choice Questions (MCQs)-Pg-271

42. The human eye forms the image of an object at its :
A. cornea
B. iris
C. pupil
D. retina

→ Retina is the part where the image is formed in the human eye.

43. 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

→ The change in focal length of an eye-lens is caused by the action of the ciliary muscles.

44. 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

→ The least distance of distinct vision for a young adult with normal vision is about 25cm.

45. Refraction of light in the eye occurs at:
A. the lens only
B. the cornea only
C. both the cornea and the lens
D. the pupil

→ Refraction of light in the eye occurs at both the cornea and the lens.

46. To focus the image of a nearby object on the retina of an eye:
A. the distance between eye-lens and retina is increased
B. the distance between eye-lens and retina is decreased
C. the thickness of eye-lens is decreased
D. the thickness of eye-lens is increased

→ To focus the image of a nearby object on the retina of an eye the thickness of eye-lens is increased.

47. The term "accommodation" as applied to the eye, refers to its ability to:
A. control the light intensity falling on the retina
B. erect the inverted image formed on the retina
C. vary the focal length of the lens
D. vary the distance between the lens and retina

→ The term "accommodation" as applied to the eye, refers to its ability to vary the focal length of the lens.

48. Which of the following controls the amount of light entering the eye ?
A. ciliary muscles
B. lens
C. iris
D. cornea

→ The amount of light entering the eye is controlled by iris.

49. The human eye possesses the power of accommodation. This is the power to:
A. alter the diameter of the pupil as the intensity of light changes
B. distinguish between lights of different colors
C. focus on objects at different distances
D. decide which of the two objects is closer.

→ The human eye possesses the power of accommodation. This is the power to focus on objects at different distances.

50. How does the eye change in order to focus on near or distant objects ?
A. The lens moves in or out
B. The retina moves in or out
C. The lens becomes thicker or thinner
D. The pupil gets larger or smaller

→ In order to change the focus on near and distance objects the lens becomes thicker or thinner.

51. Which of the following changes occur when you walk out of bright sunshine into a poorly lit room ?
A. the pupil becomes larger
B. the lens becomes thicker
C. the ciliary muscle relaxes
D. the pupil becomes smaller

→ When you walk out of bright sunshine into a poorly lit room the pupil becomes larger.

52. The size of the pupil of the eye is adjusted by:
A. cornea
B. ciliary muscles
C. optic nerve
D. iris

→ The size of the pupil of the eye is adjusted by iris.

Questions Based on High Order Thinking Skills (HOTS)-Pg-272

53. The descriptions of five kinds of images are given below:
(a) diminished and virtual (b) enlarged and real
(c) enlarged and erect (d) real and inverted
(e) virtual and the same size
Which one of these describes the image formed:
(i) on the retina of the eye?
(ii) by a magnifying glass?
(iii) by a convex driving mirror on a car?
(iv) by a plane mirror?
(v) on the screen of a slide projector?

→ (i) The image formed by the eye on the retina is described by real and inverted.
(ii) the image formed by the eye by a magnifying glass is enlarged and erect.
(iii) the image formed by a convex driving mirror of a car is diminished and virtual.
(iv) the image formed by a plane mirror is virtual and the same size.
(v) on the screen of a slide projector the image formed is enlarged and real.

54. What shape are your eye-lenses:
(a) when you look at your hand ?
(b) when you look at a distant tree ?

→ (a) the shape of the eye lens when you look at your hand is thick (more convex).
(b) When you look at a distance tree the eye-lens will be thinner (less convex).

55. Suggest how your irises help to protect' the retinas of your eyes from damage by bright light.

→ By adjusting the size of the pupil according to the intensity of light received by the eye the irises help to protect the retinas of our eyes from damage by bright light.

56A. Which parts of the eye cause rays of light to converge on the retina ?

→ Cornea and Eye-lens are the parts of the eye cause rays of light to converge on the retina.

56B. Which part causes the greatest convergence ?

→ Cornea causes the greatest convergence.

56C. Which part brings the image into sharp focus on the retina? How does it do this?

→ Eye-lens brings the image into sharp focus on the retina. It is done by changing the thickness on the lens and hence the converging power.

57. An object is moved closer to an eye. What changes must take place in the eye in order to keep the image in Sharp focus?

→ The shape of the eye-lens should must be changed by the ciliary muscles to make it thicker and increase its converging power in order to keep the image sharp and focus.

58. Why does the eye-lens not have to do all the work of converging incoming light rays?

→ The eye-lens do not have to do all the work of converging incoming light rays because cornea of the eye also converges light rays entering the eye.

59. Explain why, when it is getting dark at night, it is impossible to make out the color of cars on the road.

→ When it is getting dark at night, it is impossible to make out the color of cars on the road because the color detecting cells of the retina of eye called 'cones' do not work well in dim light.

60. Nocturnal animals (animals which sleep during the day and come out at night) tend to have wide pupils and lot of rods in their retinas. Suggest reasons for this.

→ Nocturnal animals (animals which sleep during the day and come out at night) tend to have wide pupils and lot of rods in their retinas because wide pupils allow more light to enter the eye during night. The reason behind is, Rod cells in the retina are sensitive to dim light and hence help to see in properly at night.

Very Short Answer Type Questions-Pg-279

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

→ The common defects of vision is myopia. The type of lens used to correct it is concave lens.

2. Name the defect of vision in a person;
(a) whose near point is more than 25 cm away.
(b) whose far point is less than infinity

→ (a) The defect of vision in a person whose near point is more than 25 cm away is hypermetropia.
(b) The defect of vision in a person whose far point is less than infinity is myopia.

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

→ (a) Myopia is the defect of vision which can be rectified by using a concave lens.
(b) Hypermetropia is the defect of vision which can be rectified by using a convex lens.

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

→ (a) The type of lens which is used to correct hypermetropia is convex lens.
(b) The type of lens which is used to correct myopia is concave lens.

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

→ (a) The other name for myopia is near sightedness.
(b) The other name for hypermetropia is far sightedness.

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

→ (a) The scientific name of short-sightedness is myopia.
(b) The scientific name of long-sightedness is hypermetropia.

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

→ (a) The kind of lens which is used to correct short-sightedness is concave lens.
(b) The kind of lens which is used to correct long-sightedness is convex lens.

8. State whether the following statement is true or false : Short-sightedness can be cured by using a concave lens.

→ True, Short-sightedness can be cured by using a concave lens.

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

→ Presbyopia, 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.

→ Cataract, the defect of vision which makes the eye-lens cloudy resulting in blurred vision.

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

→ Presbyopia is the other name of old age hypermetropia.

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

→ The two defects of vision which can be corrected by using spectacles are myopia and hypermetropia.

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

→ The one defect of vision (or eye) which cannot be corrected by any type of spectacle lenses cataract.

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

→ The body part with which the terms myopia and hypermetropia are connected is eye.

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

→ The far point of a person suffering from myopia (or short-sightedness) is less then infinity.

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

→ The near point of a person suffering from hypermetropia (or long-sightedness) is farther away from the normal near point (25cm).

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?

→ (a) Your friend can read a book perfectly well but cannot read the writing on blackboard unless she sits on the front row in class she is having short-sightedness.
(b) An optician prescribe her a diverging lens.

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?

→ (a) He is suffering from hypermetropia (long-sightdness).
(b) He should use convex lens to correct this defect.

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?

→ (a) The type of defect he is suffering from is myopia.
(b) This defect can be corrected by concave lens.

20. Complete the following sentences:
(a) A short-sighted person cannot see _______ objects clearly. Short-sightedness can be corrected by using..............lenses.
(b) A long-sighted person cannot see _______ objects clearly. Long-sightedness can be corrected by using.............lenses.

→ (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-Pg-279

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

→ Myopia and Hypermetropia are the two most common defects of vision (or defects of eye). By using concave lens Myopia can be corrected and by using a convex lens hypermetropia can be corrected.

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 ?

→ By using concave lens Myopia can be corrected and by using a convex lens hypermetropia can be corrected. A person having the defects of myopia as well as hypermetropia should use spectacles having bifocal lenses in which upper part consists of concave lens and lower part consists of convex lens.

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.

→ The defect of vision which can be corrected by a converging lens is Hypermetropia.
Its ray diagram can be shown as:-

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.

→ The defect of vision which can be corrected by a diverging lens is myopia.
A ray diagram for it can be shown as :-

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

→ The defect of vision due to which a person cannot see the distance objects clearly (though he can see the nearby objects clearly) is known as myopia or short sightedness. By using concave lens this eye defect can be corrected.

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

→ The defect of vision due to which a person cannot see the nearby objects clearly (though he can see the distant object clearly) is known as hypermetropia. By using convex lens this defect of vision can be corrected.

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?

→ A person suffering from the eye-defect myopia (short-sightedness) can see clearly only up to a distance of 2 meters should must use concave lens.
Calculation of the power is given by
Here, far point of myopic eye = 2m
The object kept at infinity can be seen dearly if the image of this objects is formed at 2 m.
So, object distance,
Image distance , v = 2m.

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).

→ Convex lens is needed to rectify the defect.
Its calculation for the power of the lens is given by
The defected eye can see the nearby object kept at 25 cm clearly if the image is formed at its own near point i.e. 50 cm.
Object distance, u = –25 cm
Image distance, v = –50 cm

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?

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

→ The defect of vision due to which an old person cannot see the nearby objects clearly due to loss of power of accommodation of the eye is known as presbyopia.
The two cases are :-
1) ciliary muscles gets weaken gradually with time.
2) Eye lens loses its flexibility.

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

→ When the eye lens becomes progressively cloudy resulting in blurred vision then at that time a person is said to have developed cataract in his eye.

32. Fill in the following blanks with suitable words:
A person is short-sighted if his eyeball is too ______ Spectacles with a ______ lens are needed. A person is long-sighted if his eyeball is too ______ Spectacles with a ______ lens are needed. These focus light rays exactly on to the ______

→ 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-Pg-280

33A. 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.

→ The defect of vision due to which a person can see nearby objects clearly but cannot see distant objects clearly is known as short-sightedness.
The two of its causes are :-
1) Excessive curvature of the eye lens
2) Elongation of the eyeball.
Its ray diagram can be shown as :-
(i)

(ii) The 3rd diagram shows the correction of short-sightedness by using a lens.

33B. 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?

→ A person having short-sight cannot see objects clearly beyond a distance of 1.5 m. The nature and power of the corrective lens to restore proper vision is concave.
The calculation of the power is shown as:-
Here, far point of myopic eye = 1.5 m
The object kept at infinity can be seen dearly if the image of this object is formed at 1.6m.
So, object distance = u=x
Image distance, v = 1.5 m

34A. 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.

→ The defect of vision due to which a person cannot see the nearby objects clearly but he can see the distant objects clearly is known as long-sightedness.
The two causes of long-sightedness are:-
1) Focal length of the eye lens is too long.
2) The eyeball has become too small.
(i) The ray diagram is:-

(ii) The 3rd diagram shows the correction of long-sightedness by using a lens.

34B. 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 short � sighted?

→ If an eye has a near point distance of 0.75 m, then convex lenses should be used.
v = -75 m
u = -25 m

⇒ f = 0.375
Hence Power = 1/f = 1/0.375 = +2.67D
Hence, this eye is long-sighted.

34C. 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?