Updated on: 2026-03-31 | Author: Rahul Patil

Science & Tech Part 1 Chapter 7 Lenses Solutions

Q1. 5 cm high object is placed at a distance of 10 cm from a converging lens of focal length of 20 cm. Determine the position, size and type of the image.

Given: Focal length (f) = 20 cm,

object distance (u) = – 10 cm,

height of the object (h1) = 5 cm

To find: Image distance (v), height of the image (h2)

Formulae:

`1/"f" = 1/"v" - 1/"u"`

`"h"_2/"h"_1 = "v"/"u"`

Calculation: From formula (i),
`1/20 = 1/"v" - 1/(- 10)`
∴ `1/ "v" = 1/20 - 1/10`

`= (1 - 2)/20`

∴ `1/"v" = - 1/20`
∴ v = - 20 cm

As the image distance is negative, the image formed is virtual and on the same side of lens as that of the object.

From formula (ii),
`"h"_2/5 = (- 20)/- 10`
∴ `"h"_2 = 20/10 xx 5`
∴ h2 = 10 cm

The positive sign indicates that the image formed is erect.

Q2. For a healthy human eye, the distant point is infinite distance.

For a healthy human eye, the distant point is infinite distance- Right

Q3. _______ is a combination of two convex lenses with small focal length.

Compound microscope is a combination of two convex lenses with small focal length.

Q4. _______ times larger images can be obtained by using a simple microscope.

20 times larger images can be obtained by using a simple microscope.

Q5. A ray of light gets refracted ______ while entering the lens.

A ray of light gets refracted twice while entering the lens.

Explanation:

A ray of light gets refracted twice while passing through a lens, once while entering the lens and once while emerging from the lens.

Q6. A small hole of changing diameter at the centre of Iris is called _______.

A small hole of changing diameter at the centre of Iris is called pupil.

Q7. All distances parallel to the principal axis are measured from the _______.

All distances parallel to the principal axis are measured from the optical centre.

Q8. An object is placed at a distance of 10 cm from a convex lens of focal length 12 cm, find out at what distance image will be formed from the lens and what is the nature of image.

Given: Focal length (f) = 12 cm,

Object distance (u) = −10 cm

To find: i. Image distance (v)

ii. Nature of image

Formula: `1/"f" = 1/"v" - 1/"u"`
Calculation: From formula,
`1/12 = 1/"v" - 1/(- 10)`
∴ `1/"v" = 1/12 - 1/10`

`= (10 - 12)/120`

`= (- 2)/120`

`= - 1/60`

∴ v = - 60 cm

A negative sign indicates an image is formed on the same side as that of the object.

The image is erect, virtual and magnified.

Q9. An object is placed vertically at a distance of 20 cm from a convex lens. If the height of the object is 5 cm and the focal length of the lens is 10 cm, what will be the position, size and nature of the image? How much bigger as compared to the object?

Given: Height of the object (h1) = 5 cm,
focal length (f) = 10 cm,

distance of the object (u) = –20 cm

To find: Image distance (v), height of the image (h2), magnification (M)

Formulae:

`1/"v" - 1/"u" = 1/"f"`

Magnification (M) = `"h"_2/"h"_1 = "v"/"u"`

Calculation: From formula (i),

`1/"v" = 1/"u" + 1/"f"`

∴ `1/"v" = 1/(-20) + 1/10`
∴ `1/"v" = (- 1 + 2)/20`
∴ `1/"v" = 1/20`
∴ v = 20 cm
From formula (ii),

`"h"_2 = "v"/"u" xx "h"_1`

∴ `"h"_2 = 20/(- 20) xx 5`
∴ `"h"_2 = (- 1) xx 5`
∴ `"h"_2` = - 5 cm
∴ M = `"v"/"u" = 20/(-20)` = - 1

The negative sign of the height of the image and the magnification shows that the image is inverted and real. It is below the principal axis and is of the same size as the object.

Q10. Bifocal lens is required to correct _______ defect.

Bifocal lens is required to correct presbyopia defect.

Q11. Choose the correct option from the bracket and complete the stanza. (colour blind, actual, conical, light-sensitive, rodlike, colours) The retina in our eyes is made up of many _______ cells. These cells are shaped like a rod and like a cone. The _______ cells respond to the intensity of light and give information about the brightness or dimness of the object to the brain. The _______ cells respond to the colour and give information about the colour of the object to the brain. Brain processes all the information received and we see the _______ image of the object. Rod like cells respond to the faint light also but ______ cells do not. Some people lack conical cells responding to certain colours. These persons cannot recognize those colours or cannot distinguish between different ______. These persons are said to be ______.

Answer

The retina in our eyes is made up of many light sensitive cells. These cells are shaped like a rod and like a cone. The rodlike cells respond to the intensity of light and give information about the brightness or dimness of the object to the brain.

The conical cells respond to the colour and give information about the colour of the object to the brain. Brain processes all the information received and we see the actual image of the object. Rod like cells respond to the faint light also but conical cells do not.

Some people lack conical cells responding to certain colours. These persons cannot recognize those colours or cannot distinguish between different colours. These persons are said to be colour blind.

Q12. Complete the paragraph by choosing the right options given below. (minimum, near point, 25 cm, farthest, farthest distance, far point) The _______ distance of an object from a normal eye, at which it is clearly visible without stress on the eye, is called the minimum distance of distinct vision. The position of the object at this distance is called the _______ of the eye, for a normal human eye, the near point is at _______. The _______ distance of an object from a human eye, at which it is clearly visible without stress on the eye is called _______ of distinct vision. The position of the object at this distance is called the _______ of the eye.

The minimum distance of an object from a normal eye, at which it is clearly visible without stress on the eye, is called the minimum distance of distinct vision. The position of the object at this distance is called the near point of the eye, for a normal human eye, the near point is at 25 cm. The farthest distance of an object from a human eye, at which it is clearly visible without stress on the eye is called farthest distance of distinct vision.

The position of the object at this distance is called the far point of the eye.

Q13. Convex lens : converging : : concave lens : _______

Convex lens : converging : : concave lens : diverging

Q14. Convex magnifying glass is called divergent magnifying glass and concave magnifying glass is called converging magnifying glass.

Convex magnifying glass is called divergent magnifying glass and concave magnifying glass is called converging magnifying glass- Wrong

Q15. Distinguish between: Concave lens and Convex lens

  Concave lens Convex lens
1. It is a diverging lens because it diverges the rays falling on it. It is a converging lens because it converges the rays falling on it.
2. It is thin at the center and bulged at the edge. It is thin at the edge and bulged at the centre.
3. The image formed by concave lens is always diminished and virtual. The image formed by convex lens can be real as well as virtual. Also, the image formed can be diminished as well as magnified.
4. It has a virtual focus. It has a real focus.

Q16. Distinguish between: Farsightedness and Nearsightedness.

  Farsightedness Nearsightedness
1. This problem arises due to slight flattening of the eyeball. This problem arises due to slight elongation of the eyeball.
2. The curvature of the cornea and eye lens decreases so that the converging power becomes less. The curvature of the cornea and eye lens increases so that the converging power becomes more.
3. The distance between the eye lens and the retina decreases. The distance between the eye lens and the retina increases.
4. The near point of the eye shifts farther away from the eye. The far point of the eye shifts closer to eye.
5. A person suffering from this defect can see distant objects clearly but is unable to see nearby objects. A person suffering from this defect can see nearby objects clearly but is unable to see distant objects.
6. It can be corrected by using spectacles having convex lenses of suitable focal length. It can be corrected by using spectacles having concave lenses of suitable focal length.

Q17. Distinguish between: Myopia - Presbyopia

  Myopia Presbyopia
1. This problem of vision arises due to slight elongation of eyeball or curved lens. In this problem, the near point of aged people recedes. Also, ciliary muscles lose the capacity to change the focal length ofthe eye lens.
2. In this defect, human eye can see nearby objects distinctly but unable to see distant objects clearly. In this defect, aged people find it difficult to see nearby objects comfortably and clearly. Sometimes they are unable to see nearby as well as distant objects clearly.
3. This defect can be corrected using concave lens of suitable focal length. This defect can be corrected using bifocal lens in which concave lens is useful for distinct vision and convex lens is useful for near vision.

Q18. Due to elongation of _______ and increase in curvature of the eye lens, a person cannot see distant objects clearly.

Due to elongation of eyeball and increase in curvature of the eye lens, a person cannot see distant objects clearly.

Q19. Explain with a ray diagram the position, size, and nature of the various image formed by convex lenses. An object is at infinity.

Answer

Diagram: Refer textbook

Q20. Explain with a ray diagram the position, size, and nature of the various images formed by a convex lens. An object at 2F 1 .

Answer

Diagram: Refer textbook

Q21. Explain with a ray diagram the position, size, and nature of the various images formed by a convex lens. An object is in between F 1 and 2F 1

Answer

Diagram: Refer textbook

Q22. Explain with a ray diagram the position, size, and nature of the various images formed by a convex lens. Object at focus F 1

Answer

Diagram: Refer textbook

Q23. Explain with a ray diagram the position, size, and nature of the various images formed by a convex lens. Object between F 1 and O

Answer

Diagram: Refer textbook

Q24. Explain with a ray diagram the position, size, and nature of the various images formed by the convex lens. An object beyond 2F 1 .

Answer

Diagram: Refer textbook

Q25. Find the odd one out and give its explanation.

Answer

Cerebellum

Explanation-

Cerebellum is a part of the brain while others are parts of the eye.

Q26. Find the odd one out and give its explanation.

Answer

Kaleidoscope

Explanation-

Kaleidoscope uses mirrors while other devices make use of lenses.

Q27. Find the odd one out and give its explanation.

Answer

Size of image

Explanation-

All the other distances are measured from the optical center along the principal axis while the size of the image is measured perpendicular to the axis.

Q28. Find the odd one out and give its explanation.

Answer

Principal axis

Explanation-

Except for principal axis, all other quantities can be measured physically with scale, thread, etc.

Q29. Find the odd one out and give its explanation.

Answer

Prism

Explanation-

Prism disperses light while other optical instruments produce magnified images of objects.

Q30. Find the power of a convex lens of focal length of + 25 cm.

Given: Focal length (f) = 25 cm = 0.25 m

To find: Power of the lens (P)

Formula: P = `1/("f"("m"))`
Calculation: From formula, P = `1/0.25` = 4 D

The power of the convex lens is 4 D.

Q31. Focal length : metre : : power of lens : _______

Focal length : metre : : power of lens : dioptre

Q32. For a normal human eye the near point is at _______.

For a normal human eye the near point is at 25 cm.

Q33. For a specific glass lens f = 0.5 m. This is the only information given to the student. Which type of lens is given to him and what is its power?

power 2 D; convex lens

Q34. Give a scientific reason. Simple microscope is used for watch repairs.

Answer

When an object is put inside the focal length of a magnifying glass or simple microscope (convex lens), the larger and erect image appears on the same side of the lens as the object.

By altering the distance between the object and the lens, the image can be obtained at the shortest distance of distinct vision. Thus, a watch repairer may see the minute parts of a watch more clearly with the use of a magnifying glass (a rudimentary microscope) than with the naked eye, and without putting any strain on the eye. As a result, watch repairers utilise a magnifying glass (a basic microscope) to repair the timepieces.

Q35. Give scientific reason: One can sense colours only in bright light.

Answer

The light sensitive screen of the eye, called the retina, has two types of light sensitive cells.

Rod cells are concerned with vision in dim light and cone cells are responsible for the vision in presence of light.

Cone cells are also concerned with colour perception. They do not respond to dim light or no light.

Thus, the knowledge of the colour is possible in presence of light and not in dark.

Hence, we can identify and differentiate colours in presence of light.

Q36. Give scientific reason: We cannot clearly see an object kept at a distance less than 25 cm from the eye.

Answer

The muscles attached to the eye lens (ciliary muscles) help in fine adjustments of the focal length of the lens.

The capacity of these muscles to contract or relax to adjust the focal length (i.e., power of accommodation) has a limit.

The minimum distance of an object from a normal eye for which the eye lens can decrease its focal length to the least possible value, is 25 cm.

Hence, we cannot clearly see an object kept at a distance less than 25 cm from the eye.

Q37. Given below is a diagram showing a defect of vision. Name the defect of vision and draw an accurately labelled diagram to correct this defect.

Diagram: Refer textbook

Q38. Hypermetropia can be corrected using concave lens of proper focal length.

Hypermetropia can be corrected using concave lens of proper focal length- Wrong

Q39. Identify and explain concepts given in this diagram.

Answer

The concept upon which the diagrams are based is the persistence of vision.

Explanation:

Persistence of vision is a phenomenon that allows the image formed on the retina to remain imprinted there for `1/16`th of a second after the object is removed from the front of the eyes.

When cardboard with a cage drawn on one side and a monkey drawn on the other is tied to a thread and twisted and set into rotation, at one point, the observer gets the illusion of the monkey being inside the cage.

This happens due to persistence of vision. Before the image of the cage could disappear from the retina, an image of the monkey gets created on the retina giving the observer the impression that the monkey is inside the cage.

Q40. If each two concave lenses of focal length 30 cm are kept in contact with each other what will be the power of combination?

Given: f1 = f2 = – 30 cm = – 0.3 m

To find: Power of combination (P)

Formula: P = `1/"f" = 1/"f"_1 + 1/"f"_2`
Calculation: From formula,

P = `1/(- 0.3) + 1/(- 0.3)`

`= (- 10)/3 + (- 10)/3`

`= (- 20)/3` = - 6.7 D

Power of combination is – 6.7 D.

Q41. If focal length of a convex lens is 20 cm at what is the power of the lens?

Given: Focal length (f) = 20 cm = 0.2 m

To find: Power of the lens (P)

Formula: P = `1/("f"("m"))`
Calculation: From formula,

P = `1/0.2`

= 5 D

∴ The power of the convex lens is 5 D.

Q42. In a relaxed state, the focal length of healthy eyes is _______.

In a relaxed state, the focal length of healthy eyes is 2 cm.

Q43. In hypermetropia human eye _______.

In hypermetropia human eye can see distant objects clearly.

Q44. In Myopia the human eye _______.

In Myopia the human eye cannot see distant objects clearly.

Q45. In the convex lens if an object is placed at 2 F 1 , the image is formed at ______.

In the convex lens if an object is placed at 2 F1, the image is formed at 2 F2.

Q46. : Concave lens : : : ______

Diagram: Refer textbook

Q47. Name the capacity of the eye lens to change its focal length as per need.

Power accommodation

Q48. Nearsightedness : concave lens : : farsightedness : _______

Nearsightedness : concave lens : : farsightedness : convex lens

Q49. Nearsightedness: elongated eyeball : : farsightedness: _______

Nearsightedness: elongated eyeball : : farsightedness: flattening of eye ball

Q50. Object at 2F 1 of a convex lens : Image at 2F 2 : : Object at F 1 : _______

Object at 2F1 of a convex lens : Image at 2F2 : : Object at F1 : Image at infinity

Q51. : Object near the lens : : ______ :

Diagram: Refer textbook

Q52. Observe the given below the figure, correct it and explain and write about the concept depicted in this figure.

Answer

Diagram: Refer textbook

Q53. Observe the given figure and answer the following questions. Where is the above type of lens construction used? What type of image is formed by an objective lens? What happens instead of placing at F o if the object is placed in between O and F o ?

The above type of lens construction is used in a compound microscope.

The image formed by the objective is real, inverted, and formed at a distance within the focal length of the eyepiece.

If the object is placed in between O and Fo, a virtual image will be formed on the same side of the objective as that of the object. As a result, compound microscope will not be able to operate as expected.

Q54. Simple microscope : Number of convex lens one : : compound microscope : _______

Simple microscope : Number of convex lens one : : compound microscope : Number of convex lenses two

Q55. The following figure show the change in the shape of the lens while seeing distant and nearby objects. Complete the figures by correctly labelling the diagram.

Diagram: Refer textbook

Q56. The image formed by _______ lens is always virtual and small.

The image formed by biconcave lens is always virtual and small.

Q57. The image of an object at an infinite distance is obtained in a real and erect form through a convex magnifying glass.

The image of an object at an infinite distance is obtained in a real and erect form through a convex magnifying glass- Wrong

Q58. The image of the object in the human eye is formed on the cross screen.

The image of the object in the human eye is formed on the cross screen- Wrong

Q59. The lens of the eye is flattened when looking at nearby objects.

The lens of the eye is flattened when looking at nearby objects- Wrong

Q60. The point inside the lens on the principal axis through which light rays pass without changing their path is called _______.

The point inside the lens on the principal axis through which light rays pass without changing their path is called optical centre.

Q61. The power of the magnifying glass depends on the distance of the magnifying glass from object.

The power of the magnifying glass depends on the distance of the magnifying glass from object- Wrong

Q62. The virtual image of object seen by the eye depends on the angle subtended by the object with the eye.

The virtual image of object seen by the eye depends on the angle subtended by the object with the eye- Right

Q63. Two convex lenses of focal length 30 cm and 10 cm each are kept in contact with each other. Find the power of their combination.

Given: f1 = 30 cm = 0.30 m, f2 = 10 cm = 0.10 m

To find: Power of combination (P)

Formula: P = `1/"f" = 1/"f"_1 + 1/"f"_2`
Calculation: From formula,
P = `1/"f" = 1/0.30 + 1/0.10 = 1/0.30 + 3/0.30`

`= 4/0.30` = 13.33 D

Power of combination is 13.33 D.

Q64. Virtual image is formed by a convex lens if an object is placed _______.

Virtual image is formed by a convex lens if an object is placed between F1 and O.

Q65. Vision defect that increases distance between the lens of the eye and retina of the eye is termed as myopia.

Vision defect that increases distance between the lens of the eye and retina of the eye is termed as myopia- Right

Q66. When the incident ray is parallel to the principal axis, the refracted ray passes through the principal focus.

When the incident ray is parallel to the principal axis, the refracted ray passes through the principal focus- Right

Q67. Write an Explanation. Centre of curvature of the lens

Answer

The centres of spheres whose parts form surfaces of the lenses are called centres of curvatures of the lenses.

Q68. Write an Explanation. Farthest distance of distinct vision

Answer

The farthest distance of an object from a normal human eye, at which it is clearly visible without stress on the eye is called as farthest distance of distinct vision.

Q69. Write an Explanation. Focal length

Answer

The distance between the optical centre and principal focus of a lens is called its focal length.

Q70. Write an Explanation. Magnification

Answer

Magnification due to a lens is the ratio of the height of the image to the height of the object.

Q71. Write an Explanation. Minimum distance of distinct vision.

Answer

The minimum distance of an object from a normal human eye, at which it is clearly visible without stress on the eye is called as minimum distance of distinct vision.

Q72. Write an Explanation. Persistence of vision

Answer

The phenomenon that allows the image formed on the retina to remain imprinted there for `1/16`th of a second after the object is removed from the front of the eyes is called persistence of vision.

Q73. Write an Explanation. Power of accommodation

Answer

The capacity of the eye lens to change its focal length as per need is called as its power of accommodation.

Q74. Write an Explanation. Principal axis

Answer

The imaginary line passing through both centres of curvature is called the principal axis of the lens.

Q75. Write an Explanation. Principal focus

Answer

Rays travelling parallel to the principal axis of a lens, after refraction, converge to a point on the principal axis or diverge in such a way that they appear to be coming out of a point on the principal axis. This point is called the principal focus of the lens.

Q76. Write an Explanation. The optical centre of the lens

Answer

The point inside a lens on the principal axis, through which light rays pass without changing their paths is called the optical centre of a lens.

Q77. Write law in a given figure.

Answer

Rule I: When the incident ray is parallel to the principal axis, the refracted ray passes through the principal focus.

Q78. Write law in a given figure.

Answer

Rule II: When the incident ray passes through the principal focus, the refracted ray is parallel to the principal axis.

Q79. Write law in a given figure.

Answer

Rule III: When the incident ray passes through the optical centre of the lens, it passes without changing its direction.

Q80. Write scientific reason. A convex lens is called a converging lens.

Answer

When light rays parallel to the principal axis are incident on a convex lens, they converge to a point on the principal axis.

Due to its ability to collect rays together, a convex lens is called a converging lens.

Q81. Write scientific reason. Adults need bifocal lens spectacle.

Answer

The ability of the ciliary muscles near the eye lens to change the focal length of the lens decreases with age.

As a result, sometimes adults suffer from both nearsightedness and farsightedness.

Therefore, adults need bifocal lens spectacle.

Q82. Write scientific reason. Farsightedness, this defect can be corrected by using convex lens.

Answer

Farsightedness arises due to slight flattening of the eyeball causing in decrease in the distance between the eye lens and the retina or due to decrease in the curvature of the cornea and eye lens resulting in decreased converging power.

As a result, light from a distant object arriving at the eye-lens gets converged at a point behind the retina.

When a convex lens of suitable focal length is placed between the eye and object, due to the converging effect of the convex lens, the image gets focussed on the retina.

Thus, farsightedness can be corrected by using spectacles with a convex lens.

Q83. Write scientific reason. Nearsightedness, this defect can be corrected by using spectacles with concave lens.

Answer

Nearsightedness arises due to slight elongation of the eyeball causing an increase in the distance between the eye lens and the retina or due to an increase in the curvature of the cornea and eye lens resulting in increased converging power.

As a result, light from a distant object arriving at the eye-lens gets converged at a point in front of the retina.

When a concave lens of suitable focal length is placed between the eye and object, due to the diverging effect of the concave lens, the image gets focussed on the retina.

Thus, nearsightedness can be corrected by using spectacles with a concave lens.

Q84. Write scientific reason. Presbyopia effect is more common in people over 40 years of age.

Answer

Focusing power of the eye lens decreases with age.

Also, with progressing age, ciliary the muscles near the eye lens lose their ability to adjust the focal length of the lens.

This defect of vision is called as presbyopia.

As the defect is prnounced by old age, it is more common in people over 40 years of age.

Q85. Write scientific reason. The movie cannot be enjoyed if seat of a viewer is too close to the screen in the cinema.

Answer

The nearer someone is sitting to the screen of the cinema hall; the more is the intensity of light falling on the eye.

Thus, the pupil has to contract a lot, which causes a strain in the eye.

Hence, one cannot enjoy watching a movie from a very short distance from the screen in a cinema hall.

Q86. Write the function of the human eye and label parts of the figure given below.

Answer

Double convex transparent crystalline lens: It provides small adjustments of the focal length to focus the image. This lens creates a real and inverted image of an object on the screen inside the eye.

Optic nerve: The sensitive cells on the retina get excited when light falls on them and generate electric signals. The optic nerve carries these signals to the brain.

Retina: It is the screen inside the human eye which contains light-sensitive cells. Image of an object is formed on this screen.

Pupil: The pupil contracts or widens depending on the intensity of light incident on the eye.

Q87. Write the name. The defect of eye occurring due to ageing

Answer

The defect of eye occurring due to ageing- Presbyopia

Q88. Write the name. The distance between focus and optical centre.

Answer

The distance between focus and optical centre- Focal length

Q89. Write the name. The fleshy screen behind cornea.

Answer

The fleshy screen behind cornea- Iris

Q90. Write the name. The imaginary line passing through two optical centres of lens

Answer

The imaginary line passing through two optical centres of lens- Principal axis

Q91. Write the name. The lens used in simple microscope.

Answer

The lens used in simple microscope- Convex

Q92. Write the name. The part of human eye that transmits electrical signals to the brain.

Answer

The part of human eye that transmits electrical signals to the brain- Optic nerve

Q93. Write the name. The persons which are unable to distinguish between different colours

Answer

The persons which are unable to distinguish between different colours- Colour blind

Q94. Write the name. The screen with light sensitive cells in human eye.

Answer

The screen with light sensitive cells in human eye- Retina

Q95. Write the name. The sensation on the retina persists for a while is

Answer

The sensation on the retina persists for a while is- Persistence of vision

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