NCERT Exemplar Solution for CBSE Class 10 Science: Light- Reflection and Refraction
Multiple Choice Questions (MCQs)
Question. 1 Which of the following can make a parallel beam of light when light from a point source is incident on it?
(a) Concave mirror as well as convex lens
(b) Convex mirror as well as concave lens
(c) Two plane mirrors placed at 900 to each other
(d) Concave mirror as well as concave lens
Answer. (a)
Explanation: When a point light source is placed at the focus of concave mirror then it all light rays after reflection through mirror will become parallel to the principal axis.
When this point light source is placed at the focus of convex lens then after falling on convex lens it will become parallel to the principal axis.
Question. 2 A 10 mm long awl pin is placed vertically in front of a concave mirror. A 5 mm long image of the awl pin is formed at 30 cm in front of the mirror. The focal length of this mirror is
(a) ‒ 30 cm
(b) ‒ 20 cm
(c) ‒ 40 cm
(d) ‒ 60 cm
Answer. (b)
Explanation: Here, size of object = O = + 10.0 mm = + 1.0 cm (as, 1 cm = 10 mm)
Size of Image size = I = 5.0 mm = 0.5 cm
Image distance, v = − 30 cm (as image is real)
Let, object distance = u
Focal length, f =?
NCERT Solutions for CBSE Class 10 Science
Question. 3 Under which of the following conditions, a concave mirror can form an image larger than the actual object?
(a) When the object is kept at a distance equal to its radius of curvature
(b) When object is kept at a distance less than its focal length
(c) When object is placed between the focus and centre of curvature
(d) When object is kept at a distance greater than its radius of curvature
Answer. (c)
Explanation: A concave mirror can form an image larger than the actual object when the object is placed between the focus and centre of curvature.
Image formation is shown below, where object (AB)is placed between the focus and centre of curvature for which image (A’B’) is obtained behind the mirror.
Question. 4 Figure shows a ray of light as it travels from medium A to medium B. Refractive index of the medium B relative to medium A is
Answer. (a)
Explanation: Here, angle of incidence = i = 60o
Angle of refraction = r = 450
Question. 5 A light ray enters from medium A to medium B as shown in the figure. The refractive index of medium B relative to A will be
(a) Greater than unity
(b) Less than unity
(c) Equal to unity
(d) Zero
Answer. (a)
Explanation: Since light rays in medium B goes towards normal. So it has greater restrictive index and lesser velocity of light with respect to medium A. So refractive index of medium B with respect to medium A is greater than unity.
Question. 6 Beams of light are incident through the holes A and B and emerge out of box through the holes C and D respectively as shown in the figure. Which of the following could be inside the box?
(a) A rectangular glass slab
(b) A convex lens
(c) A concave less
(d) A prism
Answer. (a)
Explanation: In the given figure, emergent light rays are parallel to the direction of incident light rays. Out of the given options only a rectangular glass slab can change the path of light ray in such a way that emergent rays are parallel to the incident rays.
Question. 7 A beam of Light is incident through the holes on side A and emerges out of the holes on the other face of the box as shown in the figure. Which of the following could be inside the box?
(a) Concave lens
(b) Rectangular glass slab
(c) Prism
(d) Convex lens
Answer. (d)
Explanation: In the given diagram, parallel rays converge at a point and emerges from face B. So, there will be a convex lens inside the box.
Question. 8 Which of the following statements is 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 power having a focal length – 0.25 m
Answer. (a)
Explanation: The power (P) of a lens of focal length (f) is given by P = 1/f , where f is the focal length meter and power in dioptre.
Now, P = 1/f ⟹ f = 1/P = 1/4 = 0.25 m.
Question. 9 Magnification produced by a rear view mirror fitted in vehicles
(a) is less than one
(b) is more than one
(c) is equal to one
(d) can be more than or less than one depending upon the position of the object in front of it.
Answer. (a)
Explanation: Convex mirror is used as rear view mirror in vehicles. It forms virtual, erect and diminished images of the objects.
As, magnification is ratio of height of image to the height of the object, hence, magnification produced by a rear view mirror fitted in vehicles is less than one.
Question. 10 Rays from sun converge at a point 15 cm in front of a concave mirror. Where should an object be placed so that size of its image is equal to the size of the object?
(a) 15 cm in front of the mirror
(b) 30 cm in front of the mirror
(c) Between 15 cm and 30 cm in front of the mirror
(d) More than 30 cm in front of the mirror
Answer. (b)
Explanation: Sun is at infinite distance from the surface of the earth.
Now light rays from sun rays after reflection from concave mirror will converge at focus.
It is given that sunrays converge at 15 cm. So, focal length of the mirror = f = 15 cm.
In case of a concave mirror, the size of image and object will be same if the object is placed at 2f. Hence, in this case object must be place at 2f or 2 × 15 = 30 cm.
Question. 11 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
Answer. (b)
Explanation: A convex mirror forms virtual, erect and diminished image of the objects. So, out of given options, distant tall building can definitely be seen by using a convex mirror.
Question. 12 In torches, search lights and headlights of vehicles, the bulb is placed
(a) between the pole and the focus of the reflector
(b) very near to the focus of the reflector
(c) between the focus and centre of curvature of the reflector
(d) at the centre of curvature of the reflector
Answer. (b)
Explanation: In torches, search lights and headlights of vehicles, the bulb is placed very near to the focus of the reflector (or concave mirror).
It is due the reason that light rays after falling on reflecting surface become parallel to the principal axis of the mirror, and a concentrated beam of light is obtained.
Question. 13 The laws of reflection hold good for
(a) plane mirror only
(b) concave mirror only
(c) convex mirror only
(d) all mirrors irrespective of their shape
Answer. (d)
Explanation: The laws of reflection holds good for light reflected form any smooth surface i.e., all mirrors regardless of its shape.
Question. 14 The path of a ray of light coming from air passing through a rectangular glass slab traced by four students shown as A, B, C and D in the figure. Which one of them is correct?
(a) A
(b) B
(c) C
(d) D
Answer. (b)
Explanation: When a light ray is passed through a glass slab obliquely then, emergent ray will be parallel to the direction of the incident ray.
Question. 15 You are given water, mustard oil, glycerine and kerosene. In which of these media, a ray of light incident obliquely at some angle would bend the most.
(a) Kerosene
(b) Water
(c) Mustard oil
(d) Glycerine
Answer. (d)
Explanation: Out of the given four materials, the refractive index of glycerine is highest. So, greatest deviation of incident light ray is observed in case of glycerine.
Question. 16 Which of the following ray diagrams is correct for the ray of light incident on a concave mirror as shown in figure?
(a) Fig. A
(b) Fig. B
(c) Fig. C
(d) Fig. D
Answer. (d)
Explanation: Ray of light going parallel to the principal axis towards the mirror after reflection will pass through the focus. So, figure D is correct.
Question. 17 Which of the following ray diagrams is correct for the ray of light incident on a lens shown in figure?
(a) Fig. A
(b) Fig. B
(c) Fig. C
(d) Fig. D
Answer. (a)
Explanation: When a light ray passing through focus will fall on convex lens then, the emergent ray of light will become parallel to the principal axis. So, figure A is correct.
Question. 18 A child is standing in front of a magic mirror. She finds the image of her head bigger, the middle portion of her body of the same size and that of the legs smaller. The following is the order of combinations for the magic mirror from the top.
(a) Plane, convex and concave
(b) Convex, concave and plane
(c) Concave, plane and convex
(d) Convex, plane and concave
Answer. (c)
Explanation: Her head appears large; it means concave mirror of large focal length is used at the top.
Her middle body appears of the same size in the mirror. It means plane mirror is used in the middle.
Her legs appear smaller. It means convex mirror is used below plane mirror. Hence, option (c) is the correct combination.
Question. 19 In which of the following, the image of an object placed at infinity will be highly diminished and point sized?
(a) Concave mirror only
(b) Convex mirror only
(c) Convex lens only
(d) Convex mirror, convex mirror, concave lens and convex lens
Answer. (d)
Explanation: The incident rays coming from an object placed at infinity will be parallel to the principal axis and the rays parallel to the principal axis, after reflection/refraction by concave mirror, convex mirror, concave lens and convex lens, will pass or appear to pass through the principal focus. Hence. Image will be highly diminished and point sized.
Short Answer type Question
Question. 20 Identify the device used as a spherical mirror or lens in following cases. When the image formed is virtual and erect in each case.
(a) Object is placed between device and its focus, image formed is enlarged and behind it.
(b) Object is placed between the focus and device, image formed is enlarged and on the same side as the of the object
(c) Object is placed between infinity and device, image formed is diminished and between focus and optical centre on the same side as that of the object.
(d) Object is placed between infinity and device, image formed is diminished and between pole and focus, behind it.
Answer.
(a) Concave mirror is used.
Question. 21 Why does a light ray incident on a rectangular glass slab immersed in any medium emerges parallel to itself? Explain using a diagram.
Answer.
As light rays enter from any other medium to glass, it deviates from its path through certain angle (). When this light ray exits from the other face of glass slab to the medium from which it is coming, then the it again deviate with the same angle but in opposite sense.
Hence, a light ray incident on a rectangular glass slab immersed in any medium emerges parallel to itself.
CBSE Class 10 Science Syllabus 2017-2018
Question. 22 A pencil when dipped in water in a glass tumbler appears to be bent at the interface of air and water. Will the pencil appear to be bent to the same extent, if instead of water we use liquids like, kerosene or turpentine? Support your answer with reason.
Answer.
A pencil when dipped in water in a glass tumbler appears to be bent at the interface of air and water due to phenomenon of refraction of light.
As we know, different materials have different refractive index. So, the amount of refraction for different media will be different. Hence, pencil will appear to be bent more or less depending on the refractive index of the medium.
Question. 23 How is the refractive index of a medium related to the speed of light? Obtain an expression for refractive index of a medium with respect to another in terms of speed of light in these two media?
Answer.
Question. 24 Refractive index of diamond with respect to glass is 1.6 and absolute refractive index of glass is 1.5. Find out the absolute refractive index of diamond.
Answer.
Let, refractive index of diamond with respect to air = nDA = absolute refractive index of diamond,
refractive index of glass with respect to air = nGA = absolute refractive index of glass,
refractive index of diamond with respect to glass = nDG
Given,
NDG = 1.6 and nGA = 1.5
NCERT Solutions for CBSE Class 10 Science
Question. 25 A convex lens of focal length 20 cm can produce a magnified virtual as well as real image. Is this a correct statement? If yes, where shall the object be placed in each case for obtaining these images?
Answer.
The statement is correct as convex lens of focal length 20 cm can produce magnified virtual and real image both.
The object should be placed between focus F and optical centre O (i.e., a distance less than 20 cm from the lens) for magnified, virtual and erect image.
The object should be placed between F and 2F (i.e., at a distance between 20 cm to 40 cm) for real, inverted and enlarged image.
Question. 26 Sudha finds out that the sharp image of the window pane of her science laboratory is formed at a distance of 15 cm from the lens. She now tries to focus the building visible to her outside the window instead of the window pane without disturbing the lens. In which direction will she move the screen to obtain a sharp image of the building? What is the approximate focal length of this lens?
Answer.
She must move the screen towards the pole lens to obtain a sharp image of the building.
It is because window pane is lying beyond 2F and convex lens forms its image, on the other side between F and 2F.
When she tries to focus the building visible to her outside the window (which is at infinity) then, the image of the building is formed at the focus.
The approximate focal length of this lens is 15 cm.
Question. 27 How are power and focal length of a lens related? You are provided with two lenses of focal length 20 cm and 40 cm, respectively. Which lens will you use to obtain more convergent light?
Answer.
The power (P) of a lens is related to its focal length (f) by following relation:
Lens of higher power or smaller focal length will converge the light rays more. So, the lens of focal length 20 cm will be used.
Question. 28 Under what condition in an arrangement of two plane mirrors, incident ray and reflected ray will always be parallel to each other, whatever may be angle of incidence. Show the same with the help of diagram.
Answer.
When two plane mirrors are placed at right angle with each other, then the incident ray and reflected ray will always be parallel to each other, irrespective of the angle of incidence.
Question. 29 Draw a ray diagram showing the path of rays of light when it enters with oblique incidence (i) from air into water; (ii) from water into air.
Answer.
(i) Ray diagram showing the path of rays of light when it enters with oblique incidence from air into water
(ii) Ray diagram showing the path of rays of light when it enters with oblique incidence from water into air.
Long Answer Type Question
Question. 30 Draw ray diagrams showing the image formation by a concave mirror when an object is placed.
(a) Between pole and focus of the mirror
(b) Between focus and centre of curvature of the mirror
(c) At centre of curvature of the mirror
(d) A little beyond centre of curvature of the mirror
(e) At infinity
Answer.
(a) The image formation by a concave mirror when an object is placed between pole and focus of the mirror
(b) The image formation by a concave mirror when an object is placed between focus and centre of curvature of the mirror
(c) The image formation by a concave mirror when an object is placed between focus and centre of curvature of the mirror at centre of curvature of the mirror
(d) The image formation by a concave mirror when an object is a little beyond centre of curvature of the mirror.
(e) The image formation by a concave mirror when the object is at infinity.
Question. 31 Draw ray diagrams showing the image formation by a convex lens when an object is placed
(a) Between optical centre and focus of the lens
(b) Between focus and twice the focal length of the lens
(c) At twice the focal length of the lens
(d) At infinity
(e) At the focus of the lens
Answer.
(a) The enlarged, virtual and erect image forms beyond 2F1 in the same side of object when the object is placed between optical centre and focus F1 of the lens.
(b) The enlarged, real and inverted image forms beyond focus 2F2 on the other side of the object when the object is placed between focus F1 and twice the focal length of the lens.
(c) The real and inverted image of equal to the size of object forms at focus 2F2 on the other side of the object when the object is placed at twice the focal length of the lens.
(d) The real, inverted and highly reduced image forms at focus F2 on the other side of the object when the object is placed at infinity.
(e) The real, inverted and highly magnified image forms at infinity on the other side of the object when the object is placed at the focus of the lens.
Question. 32 Write laws of refraction. Explain the same with the help of ray diagram, when a ray of light passes through a rectangular glass slab.
Answer.
Laws of refraction of light are
- The incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence, all lie in the same plane.
- The ratio of sine of angle of incidence to the sine of angle of refraction is constant, for the light of a given wavelength and for the given pair of media. This law is also known as Snell’s law of refraction. The constant is also known as refractive index.
The ray diagram is as shown below:
In the glass slab, the emergent rays are parallel to the incident ray because the extant of bending of the ray of light at the opposite parallel faced of rectangular glass slab are equal and opposite, so that emergent ray is parallel to incident ray.
Question. 33 Draw ray diagrams showing the image formation by a concave lens when an object is placed
(a) at the focus of the lens
(b) between focus and twice the focal length of the lens
(c) beyond twice the focal length of the lens
Answer.
(a) The image formation by a concave lens when an object is at the focus of the lens
(b) The image formation by a concave lens when an object is between focus and twice the focal length of the lens
(c) The image formation by a concave lens when an object is beyond twice the focal length of the lens
Question. 34 Draw ray diagrams showing the image formation by a convex mirror when an object is placed
(a) at infinity
(b) at finite distance from the mirror
Answer.
(a) The ray diagram is as follows
(b) The ray diagram is given below
Question. 35 The image of a candle flame formed by a lens is obtained on a screen placed on the other side of the lens. If the image is three times the size of the flame and the distance between lens and image is 80 cm, at what distance should the candle be placed from the lens? What is the nature of the image at a distance of 80 cm and the lens?
Answer.
Question. 36 Size of image of an object by a mirror having a focal length of 20 cm is observed to be reduced to 1/3rd to its size. At what distance, the object has been placed from the mirror? What is the nature of the image and the mirror?
Answer.
Here, it is not given whether the lens is concave of convex. So, we will do calculations for both types of mirror.
For concave mirror:
Focal length, f = ‒ 20 cm
Magnification, m = −1/3
Since, magnification, m = − v/u
Magnification, m = −1/3 = − v/u
⟹ v = u/3
Hence, object must be placed at a distance 40 cm from the pole of convex mirror to form erect , virtual, and diminished image.
Question. 37 Define power of a lens. What is its unit? One student uses a lens of focal length 50 cm and another of ‒ 50 cm. What is the nature of the length and its power used by each of them?
Answer.
Power of lens is defined as the ability of a lens to bend the light rays. It is given by the reciprocal of focal length of the lens in meter. Its unit is dioptre.
Focal length = f = 50 cm, then P = 100/f = 100/ 50 = 2D, lens is convex.
If focal length = f = ‒ 50 cm, then P = 100/f = 100/ −50 = −2D, lens is concave.
Question. 38 A student focused the image of a candle flume on a white screen using a convex lens. He noted down the position of the candle, screen and the lens as under
Position of candle = 12.0 cm
Position of convex tens = 50.0 cm
Position of the screen = 88.0 cm
(i) What is the focal length of the convex tens?
(ii) Where will the image be formed, if he shifts the candle towards the lens at a position of 31.0 cm?
(iii) What will be the nature of the image formed, if he further shifts the candle towards the lens?
(iv) Draw a ray diagram to show the formation of the image in case as said above.
Answer.
Let f be the focal length of the convex lens.
The distance of object should be measured from pole of the lens.
Distance of candle (or object) = Position of convex lens – Position of candle = 50 – 12 = 38 cm.
Now, by sign convention, distance of candle (or object) = u = – 38 cm
Similarly, distance of candle’s image = position of the screen – position of convex lens = 88 – 50 = 38 cm
By sign convention, distance of candle’s image = v = + 38 cm
The focal length of the convex lens is 19 cm.
(ii) When the candle is shifted towards the lens at a position of 31.0 cm.
Then, new object distance = position of convex lens c position of candle = 50 – 31 = 19
By sign convention, u = – 19 cm.
Now, focal length of the convex lens = 19 cm. It means, the candle lays at the focus of lens, hence its image is formed at infinity.
(iii) When he further shifts the candle towards the lens. This means candle lies between optical centre and focus of convex lens, so, magnified, virtual and erect image of the candle will be formed.
(iv) The ray diagram of image formation is given below:
