A abampere (aA) The unit of electric current in the CGSeniu system, defined as that current that, if flowing through two parallel conductors of negligible cross section and infinite length, placed 1 cm apart in vacuo, would produce on each conductor a force of 1 dyne per centimeter of length. 1 abampere = 1 abcoulomb/s = r statampere (where c = speed of light in cm/s) = 10 ampere. aberration Imperfect image formation due to geometric imperfections in the optical elements of a system ablation 1 . The wasting of glacier ice by any process (calving, melting, evaporation, etc.). 2. The shedding of molten material from the outer sur- face of a meteorite or tektite during its flight through the atmosphere. absolute age The age of a natural substance, of a fossil or living organism, or of an artifact, obtained by means of an absolute dating method. See absolute dating method. absolute density Density in kg/m' or, more commonly, in g/cm\ both at STP. Cf. density, relative density abso
Light – Reflection and Refraction class 10 NCERT PHYSICS MCQ & SAQ
Light – Reflection and Refraction class 10 NCERT solution:
1. Define the principal focus of a concave mirror.
Answer
Light rays that are parallel to the principal axis of a concave mirror converge at a specific point on its principal axis after reflecting from the mirror. This point is known as the principal focus of the concave mirror.
2. The radius of curvature of a spherical mirror is 20 cm. What is its focal length?
Answer
Radius of curvature, R= 20 cm
Radius of curvature of a spherical mirror = 2 × Focal length (f)R= 2f
f= R/2 = 20 / 2 = 10
Hence, the focal length of the given spherical mirror is 10 cm.
3. Name the mirror that can give an erect and enlarged image of an object.
► Concave Mirror.
4. Why do we prefer a convex mirror as a rear-view mirror in vehicles?
Answer
We prefer a convex mirror as a rear-view mirror in vehicles because it gives a wider field of view, which allows the driver to see most of the traffic behind him. Convex mirrors always form a virtual, erect, and diminished image of the objects placed in front of it.
Page No: 171
1. Find the focal length of a convex mirror whose radius of curvature is 32 cm.
Answer
Radius of curvature, R= 32 cm
Radius of curvature = 2 × Focal length (f)
R= 2ff= R/2 = 32/2 = 16
Hence, the focal length of the given convex mirror is 16 cm.
2. A concave mirror produces a three times magnified (enlarged) real image of an object placed 10 cm in front of it. Where is the image located?
Answer
Magnification produced by a spherical mirror is given by the relation,
Object distance, u= - 10 cm
v = 3 × ( - 10) = - 30 cm
Here, the negative sign indicates that an inverted image is formed at a distance of
30 cm in front of the given concave mirror.
Page No: 176
1. A ray of light traveling in air enters obliquely into water. Does the light beam bend towards the normal or away from the normal? Why?
Answer
The ray of light bends towards the normal. When a ray of light enters from an optically rarer medium (having low refractive index) to an optically denser medium (having high refractive index), its speed slows down and it bends towards the normal. Since water is optically denser than air, a ray of light entering from air into water will bend towards the normal.
2. Light enters from air to glass having a refractive index of 1.50. What is the speed of light in the glass? The speed of light in vacuum is 3 x 108 ms-1.
Answer
Refractive index of a medium, nm = Speed of light in vacuum/Speed of light in the medium
Speed of light in vacuum, c = 3 × 108 ms-1
Refractive index of glass, ng = 1.50
Speed of light in the glass, v = Speed of light in vacuum/ Refractive index of glass
= c/ng
=3 × 108/1.50 = 2x 108 ms-1.
3. Find out, from the Table, the medium having the highest optical density. Also find the medium with the lowest optical density.
Answer
Highest optical density = Diamond
Lowest optical density = Air
Optical density of a medium is directly related to the refractive index of that medium. A medium which has the highest refractive index will have the highest optical density and vice-versa.
It can be observed from table 10.3 that diamond and air respectively have the highest and lowest refractive index. Therefore, diamond has the highest optical density and air has the lowest optical
density.
4. You are given kerosene, turpentine and water. In which of these does the light travel fastest? Use the information given in the Table.
Answer
In water, light travels faster as compared to kerosene and turpentine because the refractive index of water is lower than that of kerosene and turpentine. The speed of light is inversely proportional to the refractive index.
5. The refractive index of diamond is 2.42. What is the meaning of this statement?
Answer
The refractive index of diamond is 2.42. This means that the speed of light in diamond will reduce by a factor of 2.42 as compared to its speed in air.
In other words, the speed of light in diamond is 1/2.42 times the speed of light in vacuum.
Page No: 184
1. Define 1 dioptre of power of a lens.
Answer
The SI unit of power of the lens is dioptre which is denoted by the letter D. 1 dioptre is defined as the power of a lens of focal length 1 meter.
2. A convex lens forms a real and inverted image of a needle at a distance of 50 cm from it. Where is the needle placed in front of the convex lens if the image is equal to the size of the object? Also, find the power of the lens.
Answer
Since the image is real and the same size. The position of the image should be at 2F.
It is given that the image of the needle is formed at a distance of 50 cm from the convex lens. Hence, the needle is placed in front of the lens at a distance of 50 cm.
Object distance, u= - 50 cm
Image distance, v= 50 cm
Focal length = f
According to the lens formula,
3. Find the power of a concave lens of focal length 2 m.
Answer
Focal length of concave lens, f = 2 mPower of lens, P= 1/f = 1/(-2)= -0.5D
Page No: 185
Exercise
1. Which one of the following materials cannot be used to make a lens?
(a) Water
(b) Glass
(c) Plastic
(d) Clay
► (d) Clay
2. The image formed by a concave mirror is observed to be virtual, erect and larger than the object. Where should the position of the object be?
(a) Between the principal focus and the center of curvature
(b) At the center of curvature
(c) Beyond the center of curvature
(d) Between the pole of the mirror and its principal focus.
► (d) Between the pole of the mirror and its principal focus.
3. Where should an object be placed in front of a convex lens to get a real image of the size of the object?
(a) At the principal focus of the lens
(b) At twice the focal length
(c) At infinity
(d) Between the optical center of the lens and its principal focus.
► (b) At twice the focal length
4. A spherical mirror and a thin spherical lens each have a focal length of -15 cm. The mirror and the lens are likely to be
(a) both concave
(b) both convex
(c) the mirror is concave and the lens is convex
(d) the mirror is convex, but the lens is concave
► (a) both concave
Page No: 186
5. No matter how far you stand from a mirror, your image appears erect. The mirror is likely to be
(a) plane
(b) concave
(c) convex
(d) either plane or convex
► (d) either plane or convex
6. Which of the following lenses would you prefer to use while reading small letters found in a dictionary?
(a) A convex lens of focal length 50 cm
(b) A concave lens of focal length 50 cm
(c) A convex lens of focal length 5 cm
(d) A concave lens of focal length 5 cm
► (c) A convex lens of focal length 5 cm
7. We wish to obtain an erect image of an object, using a concave mirror of focal length 15 cm. What should be the range of distance of the object from the mirror? What is the nature of the image? Is the image larger or smaller than the object? Draw a ray diagram to show the image formation in this case.
Answer
Range of the distance of the object = 0 to 15 cm from the pole of the mirror.
Nature of the image = virtual, erect and larger than the object.
8. Name the type of mirror used in the following situations.
(a) Headlights of a car
(b) Side/rear-view mirror of a vehicle
(c) Solar furnace
Support your answer with reason.
Answer
(a) Concave Mirror: This is because concave mirrors can produce powerful parallel beams of light when the light source is placed at their principal focus.
(b) Convex Mirror: This is because of its largest field of view.
(c) Concave Mirror: This is because it concentrates the parallel rays of sun at principal focus.
9. One-half of a convex lens is covered with a black paper. Will this lens produce a complete image of the object? Verify your answer experimentally. Explain your observations.
Answer
The convex lens will form a complete image of an object, even if its one half is covered with black paper. It can be understood by the following two cases.
Case I
When the upper half of the lens is covered
In this case, a ray of light coming from the object will be refracted by the lower half of the lens. These rays meet at the other side of the lens to form the image of the given object, as shown in the above figure.
Case II
When the lower half of the lens is covered
In this case, a ray of light coming from the object is refracted by the upper half of the lens. These rays meet at the other side of the lens to form the image of the given object, as shown in the above figure.
10. An object 5 cm in length is held 25 cm away from a converging lens of focal length 10 cm. Draw the ray diagram and find the position, size and the nature of the image formed.
Answer
11. A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object placed from the lens? Draw the ray diagram.
Answer
Focal length of concave lens (OF1), f = - 15 cm
Image distance, v= - 10 cm
According to the lens formula,
The negative value of u indicates that the object is placed 30 cm in front of the lens. This is shown in the following ray diagram.
12. An object is placed at a distance of 10 cm from a convex mirror of focal length 15 cm. Find the position and nature of the image.
Answer
Focal length of convex mirror, f= +15 cm
Object distance, u= - 10 cm
According to the mirror formula,
The image is located at a distance 6 cm from the mirror on the other side of the mirror.
The positive and value less than 1 of magnification indicates that the image formed is virtual and erect and diminished.
13. The magnification produced by a plane mirror is +1. What does this mean?
Answer
The positive sign means the image formed by a plane mirror is virtual and erect. Since the magnification is 1 it means that the size of the image is equal to the size of the object.
14. An object 5 cm is placed at a distance of 20 cm in front of a convex mirror of radius of curvature 30 cm. Find the position, nature and size of the image.
Answer
Object distance, u= - 20 cm
Object height, h= 5 cm
Radius of curvature, R= 30 cm
Radius of curvature = 2 × Focal length
R= 2f
f= 15 cm
According to the mirror formula,
The positive value of image height indicates that the image formed is erect.
Therefore, the image formed is virtual, erect, and smaller in size.
15. An object of size 7.0 cm is placed at 27 cm in front of a concave mirror of focal length 18 cm. At what distance from the mirror should a screen be placed, so that a sharp focused image can be obtained? Find the size and the nature of the image.
Answer
Object distance, u= - 27 cm
Object height, h= 7 cm
Focal length, f = - 18 cm
According to the mirror formula,
The negative value of image height indicates that the image formed is inverted.
16. Find the focal length of a lens of power -2.0 D. What type of lens is this?
Answer
Power of lens, P= 1/f
P= -2D
f= -1/2 = -0.5 m
A concave lens has a negative focal length. Hence, it is a concave lens.
17. A doctor has prescribed a corrective lens of power +1.5 D. Find the focal length of the lens. Is the prescribed lens diverging or converging?
Answer
Power of lens, P= 1/f
P= 1.5D
f= 1/1.5 = 10/15 = 0.66 m
A convex lens has a positive focal length. Hence, it is a convex lens or a converging lens.
Light – Reflection and Refraction class 10 NCERT PHYSICS MCQ :
Question : Coin placed in a bowl when seen from a place just disappears. When water is poured into the bowl without disturbing the coin , the coin
(a) Will not be seen (c) Becomes visible again
(b) Appears above the water surface (d) Appears very much deep inside the water
Answer : C
Question : Nature of the image formed by a convex mirror is
(a) Real, inverted, diminished (c) Real , inverted , enlarged
(b) Virtual, erect, diminished (d) Virtual, erect, enlarged
Answer : B
Question : The property of a mirror used in burning a paper is
(a) Rays from an object placed at a large distance in a concave mirror after reflection forms the image at the Focus
(b) Rays from an object placed at a large distance in a convex mirror after reflection forms the image at the Focus
(c) Rays from an object placed at Focus after reflection in a concave mirror forms the image at a very large distance.
(d) Rays from an object placed between F and 2F in a concave mirror after reflection forms the image beyond the Focus
Answer : A
Question : The focal length of a concave mirror is 10cm. The position of the object that is useful for getting an enlarged image which can be caught on a screen is
(a) Placed at a distance of 5 cm. from the pole of the mirror
(b) Placed at a distance of 15 cm from the pole of the mirror
(c) Placed at a distance of 35 cm from the pole of the mirror
(d) Placed at a distance of 5 cm from the pole of the mirror
Answer : C
Question : The power of a lens is -3.5D. The lens is
(a) Convex (c) concave
(b) Plano-convex (d) Plano-concave
Answer : C
Question : Formula to find the refractive index of a medium is
(a) n=speed of light in the medium /speed of light in air
(b) n=1 /speed of light in air
(c) n=speed of light in the air/speed of light in the medium
(d) n=1 /speed of light in the medium
Answer : C
Question : In case of refraction through a glass slab
(a) Incident ray is parallel to the refracted ray
(c) Angle of incidence is equal to the angle of refraction
(b) Incident ray is parallel to the emergent ray
(d) Angle of refraction is equal to the angle of emergence
Answer : B
Question : Mirror that can be chosen to view a tall building in a small mirror is
(a) Plane mirror (c) Convex mirror
(b) Concave mirror (d) Plano-Convex mirror
Answer : C
Question : Mirror formula is
(a) 1/v-1/u =1/f (c) 1/v+1/u =1/f
(b) M=v/u (d) M=h/h’
Answer : C
Question : The mirror used by ENT specialists is
(a) Plane mirror
(b) Concave mirror
(c) Convex mirror
(d) Plano-convex mirror
Answer : B
Question : A student obtained a blurred image of an illuminated distant tower on a screen by using a convex lens In order to obtain a sharp image of the tower on the screen ,he must shift the lens
(a) towards the screen
(b) away from the screen
(c) away from the lens
(d) either towards away or near the screen
Answer : B
Question : An object AB is placed in front of a convex Lens at its principal focus The image will be formed at
(a) focus
(b) beyond C
(c) Between F & C
(d) infinity
Answer : D
Question : When an object moves closer to a concave lens, the Image formed by it shifts
(a) Away from the lens
(b) Towards the lens
(c) First away and then towards the lens
(d) First towards and then away from the lens
Answer : A
Question : When a ray of light passes from a denser medium to a rarer medium which angle is greater
(a) angle of incidence
(b) angle of refraction
(c) both
(d) none
Answer : B
1. Which of the following mirrors is used by a dentist to examine a small cavity?
(a) Convex mirror
(b) Plane mirror
(c) Concave mirror
(d) Combination of convex and concave mirror
► (c) Concave mirror
2. The distance between the object and image will be
(a) 0.25 m
(b) 1.0 m
(c) 0.5 m
(d) 0.125 m
► (c) 0.5 m
3. An object at a distance of + 15 cm is slowly moved towards the pole of a convex mirror. The image will get
(a) shortened and real
(b) enlarged and real
(c) enlarge and virtual
(d) diminished and virtual
► (d) diminished and virtual
4. An object is placed at a distance of 0.25 m in front of a plane mirror. The distance between the object and image will be
(a) 0.25 m
(b) 1.0 m
(c) 0.5 m
(d) 0.125 m
► (c) 0.5 m
5. The power of a lens is -4.0 D. what is the nature of the lens?
(a) Plane
(b) Concave
(c) Convex
(d) Plano convex
► (b) Concave
6. For a real object, which of the following can produce a real image?
(a) Plane mirror
(b) Concave mirror
(c) Concave lens
(d) Convex mirror
► (b) Concave mirror
7. The nature of the image formed by concave mirror when the object is placed between the focus (F) and center of curvature (C) of the mirror observed by us is
(a) real, inverted and diminished
(b) virtual, erect and smaller in size
(c) real, inverted and enlarged
(d) virtual, upright and enlarged
► (c) real, inverted and enlarged
8. In torches, search lights and headlights of vehicles the bulb is placed
(a) Between pole and focus
(b) Very near to the focus
(c) Between focus and center of curvature
(d) At center of curvature
► (b) Very near to the focus
9. When an object moves closer to a concave lens the image shifts
(a) away from the lens on the same side of the object
(b) toward the lens
(c) away from the lens on the other side of lens
(d) first towards and then away from the lens
► (b) toward the lens
10. A diverging lens will produce
(a) always real image
(b) always virtual image
(c) both real and virtual image
(d) none of these
► (b) always virtual image
11. Light from the Sun falling on a convex lens will converge at a point called
(a) center of curvature
(b) focus
(c) radius of curvature
(d) optical center
► (b) focus
12. Where should an object be placed in front of a convex lens to get real image of the
size of the object?
(a) At focus
(b) At 2F
(c) At Infinity
(d) Between optical center and focus.
► (b) At 2F
13. The angle of incidence i and refraction r are equal in a transparent slab when the value of i is
(a) 0°
(b) 45°
(c) 90°
(d) depend on the material of the slab
► (a) 0°
14. If a man’s face is 25 cm in front of a concave shaving mirror producing an erect image 1.5 times the size of the face, the focal length of the mirror would be
(a) 75 cm
(b) 25 cm
(c) 15 cm
(d) 60 cm
► (a) 75 cm
15. When a ray of light goes from one medium to another, there is
(a) Always a change in its speed as well as direction
(b) No change in speed and direction
(c) A change in speed but no change in direction
(d) A change in direction but constant speed.
► (a) Always a change in its speed as well as direction
16. A magnified real image is formed by a convex lens when the object is at
(a) F
(b) between F and 2F
(c) 2F
(d) only (a) and (b) both
► (d) only (a) and (b) both
17. Which one of the following materials cannot be used to make a lens?
(a) Water
(b) Glass
(c) Plastic
(d) Clay
► (d) Clay
18. When an object moves closer to the convex lens, the image formed by it shifts
(a) away from the lens
(b) towards the lens
(c) first towards and then away from the lens
(d) first away and then towards the lens
► (a) away from the lens
19. Large number of thin stripes of black paint are made on the surface of a convex lens of focal length 20 cm to catch the image of a white horse. The image will be
(a) a zebra of black stripes
(b) a horse of black stripes
(c) a horse of less brightness
(d) a zebra of less brightness
► (c) a horse of less brightness
20. You are given three media A, B and C of refractive index 1.33, 1.65 and 1.46. The medium in which the light will travel fastest is
(a) A
(b) B
(c) C
(d) equal in all three media
► (b) B
21. The refractive index of transparent medium is greater than one because
(a) Speed of light in vacuum < speed of light in transparent medium
(b) Speed of light in vacuum > speed of light in transparent medium
(c) Speed flight in vacuum = speed of light in transparent medium
(d) Frequency of light wave changes when it moves from rarer to denser medium
► (b) Speed of light in vacuum > speed of light in transparent medium
22. The nature of image formed by a convex mirror when the object distance from the mirror is less than the distance between pole and focal point (F) of the mirror would be:
(a) real, inverted and diminished in size
(b) real, inverted and enlarged in size
(c) virtual, upright and diminished in size
(d) virtual, upright and enlarged in size
► (c) virtual, upright and diminished in size
23. A concave mirror of focal length 20 cm forms an image having twice the size of an object. For the virtual position of object, the position of object will be at
(a) 25 cm
(b) 40 cm
(c) 10 cm
(d) At infinity
► (c) 10 cm
24. As light travels from a rarer to a denser medium it will have
(a) increased velocity
(b) decreased velocity
(c) decreased wavelength
(d) both (b) and (c)
► (d) both (b) and (c)
25. The image formed by a concave mirror is real, inverted and of the same size as that of the object. The position of object should be
(a) at the focus
(b) at the center of curvature
(c) between focus and center of curvature
(d) beyond center of curvature
► (c) between focus and center of curvature
26. A concave mirror of radius 30 cm is placed in the water. It’s focal length in air and water differ by
(a) 15
(b) 20
(c) 30
(d) 0
► (d) 0
27. The angle of incidence for a ray of light having zero reflection angle is
(a) 0
(b) 30°
(c) 45°
(d) 90°
► (a) 0
28. An object at a distance of 30 cm from a
the concave mirror gets its image at the same point. The focal length of the mirror is
(a) – 30 cm
(b) 30 cm
(c) – 15 cm
(d) +15 cm
► (c) – 15 cm
29. A spherical mirror and thin spherical lens have each of focal length of -15 cm. the mirror and lens are likely to be
(a) Both concave
(b) Both convex
(c) The mirror is concave and the lens is convex
(d) The mirror is convex and the lens is concave.
► (a) Both concave
30. Where should an object be placed in front of convex lens to get a real image of the
size of the object?
(a) At the principal focus of the lens.
(b) At twice the focal length
(c) At infinity
(d) Between the optical center of the lens and its principal focus.
► (b) At twice the focal length
31. The image formed by a concave mirror is observed to be virtual, erect and larger
than the object. Where should the position of the object be?
(a) Between the principal focus and the center of curvature.
(b) At the center of curvature
(c) Beyond the center of curvature
(d) Between the pole of the mirror and its principal focus.
► (d) Between the pole of the mirror and its principal focus.
32. The refractive index of water is 1.33. The speed of light in water will be
(a) 1.33 × 108 m/s
(b) 3 × 108 m/s
(c) 2.26 × 108 m/s
(d) 2.66 × 108 m/s
► (c) 2.26 × 108 m/s
33. Which of the following statements is/are true?
(a) A convex lens has 4 dioptre power having a focal length 0.25 m
(b) A convex lens has -4 dioptre power having a focal length 0.25 m
(c) A concave lens has 4 dioptre power having a focal length 0.25 m
(d) A concave lens has – 4 dioptre having a focal 0.25 m
► (c) A concave lens has 4 dioptre power having a focal length 0.25 m
34. A full length image of a distant tall building can definitely be seen by using
(a) a concave mirror
(b) a convex mirror
(c) a plane mirror
(d) both concave as well as plane mirror
► (b) a convex mirror
35. When light falls on a smooth polished surface, most of it
(a) is reflected in the same direction
(b) is reflected in different directions
(c) is scattered
(d) is refracted into the second medium
► (a) is reflected in the same direction
36. If an incident ray passes through the focus, the reflected ray will
(a) pass through the pole
(b) be parallel to the principal axis
(c) retrace its path
(d) pass through the center of curvature
► (b) be parallel to the principal axis
37. Focal length of a concave mirror is
(a) negative
(b) positive
(c) depends on the position of the object
(d) depends on the position of image
► (a) negative
38. The image formed by a convex lens can be
(a) virtual and magnified
(b) virtual and diminished
(c) virtual and of same size
(d) virtual image is not formed
► (a) virtual and magnified
39. Image formed by reflection from a plane mirror is
(a) real and inverted
(b) virtual and erect
(c) real and erect
(d) virtual and inverted
► (b) virtual and erect
40. A ray of light is traveling from a rarer medium to a denser medium. While entering the denser medium at the point of incidence, it
(a) goes straight into the second medium
(b) bends towards the normal
(c) bends away from the normal
(d) does not enter at all
► (b) bends towards the normal
41. Which mirror can produce a virtual, erect and magnified image of an object?
(a) Concave mirror
(b) Convex mirror
(c) Plane mirror
(d) Both concave and convex mirrors
► (a) Concave mirror
42. When the object is placed between f and 2f of a convex lens, the image formed is
(a) at f
(b) at 2f
(c) beyond 2f
(d) between O and f
► (c) beyond 2f
43. Assertion: The rainbow is a manmade spectrum of sunlight in the sky.
Reason: The rainbow is formed in the sky when the sun is shining and it is raining at the same time.(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
(e) Both A and R are false.
► (a) Both A and R are true and R is the correct explanation of A.
44. Assertion: Cannot see the distant object clearly.
Reason: The far point of an eye suffering from myopia is less than infinity.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
(e) Both A and R are false.
► (b) Both A and R are true but R is not the correct explanation of A.
45. Assertion: Pupil is black in color.
Reason: Pupil is black in color as no light is reflected in it.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
(e) Both A and R are false.
► (a) Both A and R are true and R is the correct explanation of A.
Light – Reflection and Refraction class 10 NCERT PHYSICS SAQ:
Q.1. State laws of reflection of light. List four characteristics of the image formed by a plane mirror. [CBSE 2011,2015,2019]
Ans. The laws of reflection of light state that:
The incident ray, the reflected ray and the normal to the surface at the point of incidence all lie in the same plane.
The angle of incidence (i) is always equal to the angle of reflection (∠r) i.e. ∠i = ∠r
Characteristics of the image formed by a plane mirror
Image formed in a plane mirror is virtual and erect.
It is the same size as the object.
The image formed is laterally inverted.
The image formed is at the same distance behind the mirror as the object in front of the mirror.
Q.2. A student, holding a mirror in his hand, directed the reflecting surface of the mirror towards the sun. He then directed the reflected light on to a sheet of paper held close to the mirror.
(a) What should he do to burn the paper?
(b) Which type of mirror does he use?
(c) Will he be able to determine the approximate value of focal length of this mirror from this activity ? Give reason and draw a ray diagram to justify your answer in this case. [CBSE 2019]
Ans. (a) The student should adjust the distance between the mirror and the paper so that solar rays are sharply focussed on the paper.
(b) The mirror is a concave mirror.
(c) The student can find the approximate focal length by measuring the distance between the paper and the mirror.
As shown in Fig. 10.29, parallel rays from the sun are focussed on the paper at point A' in the focal plane of the mirror such that PB' = f.
Q.3. A concave mirror has a focal length of 20 cm. At what distance from the mirror should a 4 cm tall object be placed so that it forms an image at a distance of 30 cm from the mirror ? Also calculate the size of the image formed. [CBSE 2019]
Ans. As per question, focal length of concave mirror f = — 20 cm, height of object h = + 4 cm and distance of image v = 30 cm.
Following two cases may arise here :
Case I: If the image formed is real then v - — 30 cm and so from mirror formula
we have
and
So the object is placed 60 cm in front of the mirror and the image is an inverted image of size 2 cm.
Case II: If the image formed is virtual then v - + 30 cm and now
and
So the object is placed at 12 cm in front of the mirror and the image is an erect image of height 10 cm.
Q.4. A 10 cm tall object is placed perpendicular to the principal axis of a convex lens of focal length 12 cm. The distance of the object from the lens is 18 cm. Find the nature, position and size of the image. [CBSE 2019]
Ans. As per question f = + 12 cm, u = - 18 cm and h = + 10 cm
As per lens form
we have
The image is formed on the opposite side of the lens at a distance of 36 cm from it. The image is a real and inverted image.
Moreover, magnification
So, the size of the image is 20 cm tall and is formed below the principal axis.
Q.5. A 5 cm tall object is placed perpendicular to the principal axis of a convex lens of focal length 20 cm. The distance of the object from the lens is 30 cm. Find the (i) position, (ii) nature, and (iii) size of the image formed. [CBSE 2004, 2006, 2014, 2019]
Ans. Here h - + 5 cm, f= + 20 cm, u = - 30 cm
(i) Using lens formula
we have
(ii) +ve sign of v means that image is being formed on the other side of the lens i.e., the image is a real image.
Q.6. A real image, 2/3 rd of the size of an object, is formed by a convex lens when the 3 objects are at a distance of 12 cm from it. Find the focal length of the lens. [CBSE 2019]
Ans. Here distance of the object from the lens u = - 12 cm and magnification of real image m = -2/3
As per relation m = v/u , we have v = mu = (-2/3) x (-12) = +8 cm
So as per lens formula
we have
Q.7. Draw a ray diagram to show refraction through a rectangular glass slab. How is the emergency ray related to the incident ray ? What is its lateral displacement ? [CBSE 2010, 2011, 2013, 2014, 2019]
Ans. A ray diagram showing refraction through a rectangular glass slab has been shown in adjoining Fig. 10.32.
The emergent ray GH is exactly parallel to the incident ray CFNM. It means that ∠r2 = ∠i1.
However, the emergent ray is laterally (sideways) displaced as compared to the original path of the light ray. In the ray diagram, the lateral displacement is GN. Its value increases by increasing the width of the glass slab.
Q.8. State the laws of refraction of light.
Explain the term absolute refractive index of a medium and write an expression to relate it with the speed of light in vacuum. [CBSE 2018]
Ans. Two basic laws of refraction of light are :
(i) The incident ray, the refracted ray and the normal to the separating surface at the point of incidence, all lie in the same plane.
(ii) The ratio of sine of the angle of incidence (i) to the sine of angle of refraction (r) is a constant. It is known as Snell’s law. Thus, according to Snell’s law
Sin i/sin r = a constant = n
Generally, the constant n is known as the absolute refractive index of a given medium. Thus, absolute refractive index of a medium is defined as the ratio of sine of angle of incidence of a light ray in air (or vaccum) to the sine of angle of refraction of the ray in given medium.
Absolute refractive index of a medium is a unitless quantity and its value is one or greater than one In terms of speed of fight, the absolute refractive index of a medium is defined as:
Absolute refractive index of a medium (re) = Speed of light in vacuum (or air) ‘ c / Speed of light in given medium ‘ v’
Q.9. What is meant by the power of a lens? Write its SI unit. A student uses a lens of focal length 40 cm and another of - 20 cm. Write the nature and power of each lens. [CBSE 2018]
Ans. The power (P) of a lens is defined as the reciprocal of its focal length (f) expressed in meters.
Thus,
The SI unit of power is dioptre (D), where 1 D = 1 m-1.
The focal length of lens used by one student, f1 = + 40 cm = + 0.4 m
∴ The lens is a convex lens and the power of lens
Again the focal length o f lens used by another student, f2 = - 20 cm = - 0.2 m
∴ The lens is a concave lens and the power of lens
Q.10. What is meant by the power of a lens? Write its SI unit. A student uses a lens of focal length 40 cm and another of -20 cm. Write the nature and power of each lens. [CBSE 2018]
Ans. Power of a Lens: The ability of a lens to converge or diverge the ray of light after refraction, is called power (P) of the lens. It is defined as the reciprocal of the focal length, i.e. P = 1/f.
The SI unit of power of a lens is ‘dioptre’. A lens of focal length 100 cm has a power of 1 dioptre = 1 m-1.
Given: fA = + 40 cm = 0.4 m, fB = -20 cm
Hence, the nature of lens A is convex with power + 2.5D and lens B is concave with power-5D.
Q.11. What is meant by the power of the lens? You have three lenses L1, L2 and L3 of power +10 D, +5 D and - 10 D respectively. State the nature and focal length of each lens. Explain which of the three lenses will form a virtual and magnified image of an object placed at 15 cm from the lens. Draw the ray diagram in support of your answer. [CBSE Sample Paper 2018]
Ans. Power of lens: It is the degree of convergence or divergence of light rays after refraction through a spherical lens is called power of lens. It is the reciprocal of its focal length.
= + 20 cm (convex lens)
When an object is placed between the focus and optical center of a convex lens, a virtual erect and magnified image is formed on the same side of the lens.
Hence, for the object distance of 15 cm, lens L2 will form the same.
Q.12. We wish to obtain an equal sized inverted image of a candle flame on a screen kept at a distance of 4 m from the candle flame.
(a) Name the type of lens that should be used.
(b) What should be the focal length of the lens and at what distance from the candle flame the lens should be placed.
(c) Draw a labeled diagram to show the image formation in this case. [CBSE 2018C]
Ans. (a) Convex lens.
(b) 2 F = 4 ⇒ f = 2 m
Distance of candle flame from the lens = 4 m.
Q.13. If the image formed by a mirror for all positions o f the object placed in front of it is always erect and diminished, what type of mirror is it ? Draw a ray diagram to justify your answer. Where and why do we generally use this type of mirror ? [CBSE 2015,2017]
Ans. Only a convex mirror always forms an erect and diminished image behind the mirror between its pole and focus point for all positions of the object placed in front of the mirror.
A ray diagram showing the image formation of an object AB is shown here.
A convex mirror is used as a rear view mirror in automobiles because it gives erect and diminished images of vehicles coming from behind. As a result, it helps the driver in having a much wider field of view.
Q.14. An object 4 cm in height, is placed at 15 cm in front of a concave mirror of focal length 10 cm. At what distance from the mirror should a screen be placed to obtain a sharp image of the object. Calculate the height of the image. [CBSE 2017]
Ans. Here distance of object u = — 15 cm, height of object h = + 4 cm and the focal length of concave mirror f = - 10 cm.
As per mirror formula
we have
Thus, a screen can be placed in front of the mirror at a distance of 30 cm from it.
∴
Thus, the image is an inverted image of height 8 cm.
Q.15. An object is placed at a distance of 30 cm from a concave lens of focal length 15 cm. List four characteristics (nature, position, etc.) of the image formed by the lens. [CBSE 2017]
Ans. (a) The image o f a given object formed by the given concave lens is (i) virtual, (ii) erect, (Hi) diminished in size, and (iv) is formed on the same side of the lens at a distance less than 15 cm from the lens.
Q.16. Write two different uses of a concave mirror. [Delhi 2017C]
Ans. (i) In a solar furnace (ii) In a shaving mirror.
Q.17. Define the power of a lens. [AI 2017C]
Ans. The ability of the lens to converge or diverge the ray of light is called the power of the lens. It is equal to the reciprocal of focal length, i.e. P = 1/f
Q.18. The magnification of an image formed by a lens is -1. If the distance of the image from the optical center of the lens is 25 cm, where is the object placed? Find the nature and focal length of the lens. If the object is displaced 15 cm towards the optical center of the lens, where would the image be formed? Draw a ray diagram to justify your answer. [Foreign 2017]
Ans. For real image, m = - 1
Therefore, m = v/u = -1
or u = -v, v = -u, - u = v = — 25
Using lens formula,
Thus, the positive focal length shows that the given lens is a convex lens of focal length 12.5 cm. If the object is now displaced 15 cm towards the optical center of the lens i.e, the object is now placed at a distance of 25 - 15 = 10 cm from the optical center.
Therefore u = - 10 cm , and f = +12.5 cm.
Using lens formula again,
or
v = -50
So, in this case, virtual image is formed on the same side of the object at a distance of 50 cm from the optical center o f the lens as shown in figure
Q.19. If the image formed by a lens for all positions of an object placed in front of it is always erect and diminished, what is the nature of this lens? Draw a ray diagram to justify your answer. If the numerical value of the power of this lens is 10 D, what is its focal length in the Cartesian system? [AI 2017]
Ans. It is a concave or diverging lens.
Or
f = -10cm.
Q.20. Define the term magnification as referring to spherical mirrors. If a concave mirror forms a real image 40 cm from the mirror, when the object is placed at a distance of 20 cm from its pole, find the focal length of the mirror. [Delhi 2017C]
Ans. Magnification of spherical mirror (m): It is equal to the ratio of size (height) o f the image to the size (height) of the object. Thus,
Given: For a concave mirror u = - 20 cm,
v = - 40 cm,
Using a mirror equation,
or
⇒
Q.21. State Snell's law of refraction of light. Express it mathematically. Write the relationship between absolute refractive index of a medium and speed of light in vacuum. [AI 2017C]
Ans. Snell’s law: The ratio of sine of angle of incidence (i.e. sin i) to the sine of angle of refraction (i.e. sin r) is always constant for the light of given color and for the given pair of media.
Mathematically, sin i/ sin r = constant = n21
The constant n21 is called the refractive index of the second medium with respect to the first medium.
Absolute refractive index of the medium is given by
Q.22. Draw ray diagrams to show the principal focus of a (i) concave mirror (ii) convex mirror. [CBSE 2016]
Ans. Ray diagrams have been shown in the following Figs. 10.11(a) and (b) respectively.
Fig. 10.11
Q.23. Complete the following diagram [Fig. 10.12]: [CBSE 2011,2013,2016]
Ans. The completed diagram is as given above in Fig. 10.13.
Q.24. Define the optical center of a spherical lens. [CBSE 2016]
Ans. Optical center of a spherical lens is a point on its principal axis, a ray of light passing through which goes undeviated along its path after refraction.
Q.25. Define a lens. [CBSE 2015,2016]
Ans. See Point Number 30 under the heading “Chapter At A Glance”
Q.26. Draw the following diagram [Fig. 10.22], in which a ray of light is incident on a concave/convex mirror, on your answer sheet. Show the path of this ray, after reflection, in each case. [CBSE 2011, 2012, 2014,2016]
Ans. The diagrams have been drawn and the path of rays after reflection have been shown:
Q.27. State two positions in which a concave mirror produces a magnified image of a given object. List two differences between the two images. [CBSE 2016]
Ans. Two positions in which a concave mirror produces a magnified image are :
(i) When an object is placed between focus point F and center of curvature C of the mirror. For this position the image is real, inverted and magnified, and the image is formed beyond the centre of curvature of the mirror.
(ii) When an object is placed between pole P and focus point F of the mirror. For this position the image is virtual, erect and magnified and the image is formed behind the mirror.
Q.28. It is desired to obtain an erect image of an object, using a concave mirror of focal length of 12 cm.
(i) What should be the range of distance of an object placed in front o f the mirror?
(ii) Will the image be smaller or larger than the object ? Draw a ray diagram to show the formation of the image in this case.
(iii) Where will the image of this object be, if it is placed 24 cm in front of the mirror? Draw a ray diagram for this situation also to justify your answer. [CBSE 2016]
Ans. It is given that the focal length of a concave mirror is 12 cm.
(i) To obtain an erect image of an object by this mirror, the object should be placed in front of the mirror between its pole and focus point, that is |u| < 1 2 cm.
(ii) The image is larger than the object. The ray diagram has been shown in Fig. 10.25.
(iii) If object be placed at 24 cm in front of the mirror then it means that the object is situated at the center of curvature [ ∵ | u | = 24 cm = 2f - 2 x 12 cm = R ] C of the given mirror. Hence as shown in Fig. 10.24 the real, inverted image of same size is formed at center of curvature C itself [ |v| - 24 cm].
Q.29. (a) Draw a ray diagram to show the formation of an image by a convex lens when an object is placed in front of the lens between its optical center and principal focus.
(b) In the above ray diagram mark the object-distance (u) and the image distance (v) with their proper signs (+ve or -ve as per the new Cartesian sign convention) and state how these distances are related to the focal length (f) of the convex lens in this case. [CBSE 2016]
Ans. (a) The ray diagram is as shown in Fig. 10.37.
(b) The object distance 'u', the image distance 'v' and the focal length 'f' of the convex lens are correlated as per relation :
While applying this formula we must specify +ve or -ve signs of u, v and f as per new Cartesian sign convention being followed.
Q.30. Find the power of a convex lens which forms a real, and inverted image of magnification -1 of an object placed at a distance of 20 cm from its optical center. [CBSE 2016]
Ans. Here u = - 20 cm and magnification m = - 1 for real and inverted image
∵ m = v/u, hence v = mu =(-1) x (-20) = + 20 cm
∴
⇒ f= 10 cm = 0.1 m
∴ Power of convex lens P =
Q.31. A divergent lens has a focal length of 20 cm. At what distance should an object of height 4 cm from the optical center of the lens be placed so that its image is formed 10 cm away from the lens? Find the size of the image also. Draw a ray diagram to show the formation of the image in the above situation. [CBSE 2016]
Ans. Here focal length of given divergent (concave) lens f = - 20 cm, height of the object h = + 4 cm and distance of image from the lens v = 10 cm. As the image formed by a concave lens is always virtual and erect, hence as per sign convention u = - 10 cm.
As per lens formula
we have
∴
⇒ u = -20 cm
Thus, the object is placed at a distance 20 cm from the lens.
Moreover
A ray diagram to show the formation of the image is given here in Fig. 10.39.
Q.32. The image of an object formed by a mirror is real, inverted and is of magnification -1. If the image is at a distance of 40 cm from the mirror, where is the object placed? Where would the image be if the object is moved 20 cm towards the mirror? State the reason and also draw a ray diagram for the new position of the object to justify your answer. [AI 2016]
Ans. Given: Magnification of spherical mirror = -1, Image
distance, v = -40 cm
Magnification, m = -v/u
⇒
Therefore, the object is placed at a distance of 40 cm in front of the spherical mirror.
Case I: when u = -40 cm and v = -40 cm,
Using mirror formula, we get
⇒
⇒
Hence the focal length of the mirror is 20 cm, and the negative focal length shows that it is a concave mirror.
The new position of the object when it moves 20 cm towards the concave mirror, u’ = - ( 4 0 - 20) = -20 cm.
Case II: u’ = - 20 Cm, f = - 20 cm, v =?
From the mirror formula,
⇒
⇒
⇒
Thus, the image is formed at infinity.
Hence when the object is moved 20 cm towards the mirror, a real, inverted and highly enlarged image is formed at infinity.
Q.33. If the image formed by the mirror for all positions of the object placed in front of it is always virtual and diminished, state the type of the mirror. Draw a ray diagram in support of your answer. Where are such mirrors commonly used and why? [AI 2015, Foreign 2016]
Ans. Convex Mirror
A convex mirror is commonly used as a rear-view mirror in vehicles because it always produces a virtual and erect image whose size is smaller than the object. Therefore, it enables the driver to see a wide field of view of the traffic behind the vehicle in a small mirror.
Q.34. If the image formed by a lens for all positions of the object placed in front of it is always virtual, erect and diminished, state the type of the lens. Draw a ray diagram in support of your answer. If the numerical value of focal length of such a lens is 20 cm, find its power in new cartesian sign conventions. [Foreign 2016]
Ans. Concave lens
(i) When an object is placed at infinity.
(ii) When an object is placed between F1 and 2 F1 .
Thus, from the above figures, it is clear that whatever be the position of the object in front of a concave lens, the image formed is always virtual, erect and diminished.
The power of the given lens is calculated as
⇒ R = - 5 D
Q.35. Name the mirror that is used by a dentist in examining teeth. [CBSE 2012,2015]
Ans. Dentists use a concave mirror to See large images of the teeth of patients.
Q.36. What is lateral displacement of a light ray passing through a glass slab? [CBSE 2014,2015]
Ans. The shifting of the light ray sideways (though in the direction of the original ray) on emergence from a rectangular glass slab is called “lateral displacement”.
Q.37. Define the power of a lens and write its SI unit. [CBSE 2011,2012,2015]
Ans. Reciprocal of the focal length of a lens, expressed in meters, is called the power of that lens. Its SI unit is 1 dioptre (1 D), where 1 D = 1 m-1.
Q.38. Name the lens which can be used as a magnifying glass. [CBSE 2015]
Ans. A convex lens can be used as a magnifying glass so as to form a magnified image of a tiny object placed near it.
Q.39. Which type of lens has a negative power ? [CBSE 2015]
Ans. A concave (diverging) lens has a negative power.
Q.40. What is the difference between a virtual image of an object formed by a convex lens and that formed by a concave lens? [CBSE 2011,2015]
Ans. 'Virtual image formed by a convex lens is always magnified but that formed by a concave lens is a diminished one.
Q.41. During its passage from one medium to another, where does a light ray change its path? [CBSE 2006,2011,2015]
Ans. During its passage from one medium to another a light ray changes its path at the boundary face separating the two media.
Q.42. The power of a lens is + 5 D. Find its focal length in meters. [CBSE 2015]
Ans. Focal length f = 1/P = +1/5 m = 0.2m
Q.43. To construct a ray diagram we use two rays of light which are so chosen that it is easy to determine their directions after reflection from the mirror. Choose these two rays and state the path of these rays after reflection from a concave mirror. Use these two rays to find the nature and position of the image of an object placed at a distance of 15 cm from a concave mirror of focal length 10 cm. [CBSE 2012, 2015]
Ans. The position of the image formed by a spherical mirror can be found by considering the following two rays:
(i) The ray incident parallel to the principal axis, after reflection, passes through the principal focus of a concave mirror.
(ii) A ray passing through the center of curvature in a concave mirror after reflection, retraces its path.
The image formation is shown in the adjacent figure. Here for every 5 cm distance we have 1 cm in the ray diagram. Here AB is the object and ATT is the real and inverted image formed on the basis of the above mentioned two rays.
Actual measurement shows that the image is formed 30 cm in front o f the concave mirror i.e., v = 30 cm.
Q.44. (a) Define the optical center of a lens. What happens when a ray of light passes through the optical center of the lens?
(b) Define principal focus and focal length of a lens. Draw a ray diagram to show the position of principal focus of a lens. [CBSE2015]
Ans. (a) The optical center of a thin lens is a point on its principal axis, a ray of light passing through which goes straight without any bending (or deviation).
In Fig. 10.33, point O is the optical center.
(b) Principal focus of a lens is a point where a light beam incident parallel to the principal axis of the lens, after refraction, actually converges to (in case of a convex lens) or diverges from (in case of a concave lens). Since a lens has two refracting surfaces, a lens has two principal foci. F1 and F2 on either side of the lens.
The distance of principal focus of a lens from its optical center is called its focal length f.
Thus, f = OF1= OF2.
Q.45. What is the magnification of the images formed by plane mirrors and why?
Ans. The magnification of the images formed by plane mirrors is 1 as the size of the image is equal to the size of the object.
Q.46. A student wants to project the image of a candle flame on a screen 80 cm in front of a mirror by keeping the candle flame at a distance of 20 cm from its pole.
(i) Which type of mirror should the student use?
(ii) Find the magnification of the image produced.
(iii) Find the distance between the object and its image.
(iv) Draw a ray diagram to show the image formation in this case and mark the distance between the object and its image. [Foreign 2015]
Ans. (i) Concave mirror, as it forms a real image on the same side of the mirror.
(ii) Magnification, The negative sign in magnification shows that the image formed is real and inverted.
(iii) Distance between the object and its image = 80 - 20 = 60 cm.
(iv) The focal length of the concave mirror is given by
∴ f= - 16 cm, R = 2f = - 32 cm
Since u = - 20 cm, it implies that the object lies between F and C, so the image is formed beyond the center of curvature as shown below:
The image is real, inverted and enlarged.
Q.47. Draw a ray diagram to show the path of the reflected ray in each of the following cases. A ray of light incident on a convex mirror.
(a) strikes at its pole making an angle θ from the principal axis.
(b) is directed towards its principal focus.
(c) is parallel to its principal axis. [Foreign 2015]
Ans.
Q.48. An object of height 5 cm is placed perpendicular to the principal axis of a concave lens of focal length 10 cm. If the distance of the object from the optical center of the lens is 20 cm, determine the position, nature and size of the image. [AI 2015]
Ans. Given: h0 = + 5 c m , f = - 10 cm, u = - 20 cm,
v =?, hi =?
Using lens formula,
⇒
∴
So, the image is formed on the same side of the object at a distance of 6.67 cm. The negative sign indicates that the image is virtual.
Also |u|< |u| so the image is diminished.
∵
or
or
So, the image is virtual, erect, diminished and of size 1.66 cm.
Light – Reflection and Refraction class 10 NCERT PHYSICS LONG QUESTION:
Q.1. An object is placed at a distance of 60 cm from a concave lens of focal length 30 cm.
(i) Use the lens formula to find the distance of the image from the lens.
(ii) List four characteristics of the image (nature, position, size, erect/inverted) formed by the lens in this case.
(iii) Draw a ray diagram to justify your answer in part (ii). [Delhi 2019]
Ans. Given u = - 60 cm, f = - 30 cm
(i) Using lens formula,
∴
∴ v = -20 cm
(ii) Nature of image : Virtual
Position of image : Between optical center and focus Of concave lens.
Size of image: Smaller than the object using
So the size of the image is one third of the object.
Erect/inverted : Erect image
(iii)
Q.2. (a) List four characteristics of the image formed by a convex lens when an object is placed between its optical center and principal focus.
(b) Size of the image of an object by a concave lens of focal length 20 cm is observed to be reduced to 1/3 rd of its size. Find the distance of the object from the lens. [CBSE2019]
Ans. (a) When an object is placed between the optical center and principal focus of a convex lens, the image formed is virtual, erect and enlarged. Moreover, the image is formed on the same side of the lens behind the object.
(b) Here magnification of the given concave lens m = +1/3 and focal length of lens f = - 20 cm.
As per relation m = v/u for a lens, we get
Therefore, as per sign convention followed, both u and v are -ve.
Using lens formula we have
⇒
So the object is placed at a distance of 40 cm from the lens.
Q.3. (a) What is the minimum number of rays required for locating the image formed by a concave mirror for an object? Draw a ray diagram to show the formation of a virtual image by a concave mirror.
(b) The linear magnification produced by a spherical mirror is+3. Analyze this value and state the (i) type o f mirror and (ii) position of the object with respect to the pole of the mirror. Draw ray diagram to show the formation of image in this cased
(c) An object is placed at a distance of 30 cm in front of a convex mirror of focal length 15 cm. Write four characteristics of the image formed by the mirror. [Delhi 2017]
Ans. (a) Two rays are required.
(b) The linear magnification produced by a spherical mirror is +3. It shows that the size of the image is three times the size of the object, image is virtual and erect and formed behind the mirror. Hence
(i) the mirror is a concave mirror, and
(ii) the object is placed between the pole and the focus of a concave mirror.
(c) The four characteristics of the image formed by the convex mirror are virtual, erect, diminished and laterally inverted.
Q.4. (a) To construct a ray diagram we use two rays which are so chosen that it is easy to know their directions after reflection from the mirror. List two such rays and state the path of these rays after reflection in case of concave mirrors. Use these two rays and draw a ray diagram to locate the image of an object placed between the pole and the focus of a concave mirror.
(b) A concave mirror produces three times magnified image on a screen. If the object is placed 20 cm in front of the mirror, how far is the screen from the object? [Delhi 2017]
Ans. (a) Rays which are chosen to construct a ray diagram for reflection are: O') A ray parallel to the principal axis and 00 A ray passing through the center of curvature of a concave mirror.
Path of these light rays after reflection:
(i) It will pass through the principal focus of a concave mirror
(ii) It gets reflected back along the same path. When an object is placed between the pole and the principal focus of a concave mirror, a virtual, erect and enlarged image is formed behind the concave mirror as shown in the figure.
(b) Given: u = — 20 cm and m = 3
Magnification, m, is given by
∴ v = - m x u
= - (- 3) (- 20 cm) = - 60 cm
Distance between the object and the screen is
= - 60 cm - (- 20 cm)
= - 40 cm.
Q.5. Analyze the following observation table showing variation of image-distance (v) with object distance (u) in case of a convex lens and answer the questions that follow, without doing any calculations:
(a) What is the focal length of the convex lens? Give reason to justify your answer.
(b) Write the serial number of the observation which is not correct. On what basis have you arrived at this conclusion?
(c) Select an appropriate scale and draw a ray diagram for the observation at S.No. 2. Also find the approximate value of magnification. [AI 2017]
Ans. (a) The focal length of the convex lens is f = 20, Cm.
Reason: Objects at S.No. 3 indicates that u - -40 cm, v = +40 cm
Thus, the object is at 2F.
Therefore, 2f - 40 cm ⇒ f= 20 cm
(b) Observation at S.No. 6 is not correct.
The value, w = - 15 cm, indicates that the object is in between the optical center and the focus (i.e., less than the focal length) of the lens and hence, the image should be on the same side as the object. Accordingly, the image distance should be negative and cannot be positive (+120 cm) as shown in the table.
(c) Ray diagram for the observation at S.No. 2 : Given: u = - 60 cm; v =+30 cm; f= 20 cm
Q.6. (a) If the image formed by a mirror for all positions of the object placed in front of it is always diminished, erect and virtual, state the type of the mirror and also draw a ray diagram to justify your answer. Write one use such mirrors are put to and why?
(b) Define the radius of curvature of spherical mirrors. Find the nature and focal length of a spherical mirror whose radius of curvature is +24 cm. [Al 2017]
Ans. (a) Convex (diverging) mirror
view mirror.
Reason: (i) It always produces a virtual and erect image.
(ii) The size of the image formed is smaller than the object.
Therefore, it enables the driver to see a wide field view of the traffic behind the vehicle in a small mirror.
(b) Radius of Curvature: The separation between the pole and the center of curvature or the radius of the hollow sphere, of which the mirror is a part, is called radius of curvature (R), i.e., PC = R.
Since the focal length of the mirror is +24 cm. It indicates that the given spherical mirror is convex/diverging.
As R = 2f = 24 cm
Therefore, f = +12 cm
Q.7. (a) Draw labeled ray diagrams for each of the following cases to show the position, nature and size of the image formed by a convex lens when the object is placed. (i) between its optical center (O) and principal focus (F)
(ii) between F and 2 F
(b) How will the nature and size of the image formed in the above two cases, (i) and (ii) change, if the convex lens is replaced by a concave lens of same focal length? [Delhi 2017C]
Ans. A convex lens of focal length 'f' can form
(i) a magnified and erect image only when the object is placed between its focus 'F' and optical center ‘O’ of the lens.
(ii) a magnified and inverted image when an object is placed in the following positions: Between F1 and 2F1
(b) Whatever be the position o f the object as given in case (i) and (ii),the image formed by the concave lens is always virtual, erect and diminished.
Q.8. State the laws that are followed when light is reflected by spherical mirrors. Draw a ray diagram to show the formation of an image of an object placed in front of a convex mirror. List two characteristics of the image formed. Briefly explain one use of a convex mirror. [AI 2017C]
Ans. The two laws of reflection of light are:
(i) The angle of incidence is equal to the angle of reflection.
(ii) The incident ray, the normal to the reflecting surface at the point of incidence and the reflected ray from that point, all lie in the same plane.
(i) Image formed is behind the mirror between pole (P) and focus (F).
(ii) Virtual, erect and diminished images are formed.
(ii) Rear-view mirror of vehicles: Convex mirror Convex mirror is used because it always produces a virtual and erect image whose size is smaller than the object. Therefore, it enables the driver to see a wide field view of the traffic behind the vehicle in a small mirror.
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