Class 10 Chapter 10 Light : Reflection and Refraction

Multiple Choice Questions

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 90° to each other

(d) Concave mirror as well as concave lens

Answer :  (a) Concave mirror as well as convex lens.

[ Both a concave mirror and a convex lens have the ability to converge parallel rays of light to a point focus. In the case of a concave mirror, parallel rays converge after reflection, while in the case of a convex lens, parallel rays converge after refraction. Therefore, both a concave mirror and a convex lens can create a parallel beam of light when light from a point source is incident on them.]

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) – 20 cm

[ Here,   ,  ,

We have ,   

 

Using mirror formula , we have   

  ]

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) When the object is placed between the focus and the centre of curvature.

[ In a concave mirror, when an object is located between the focus (F) and the centre of curvature (C), the mirror forms a magnified and virtual image that is larger than the actual object. This is a specific case where the concave mirror exhibits magnification greater than one.]

4. Figure 10.1 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

(a) 

(b) 

(c) 

(d) 

Fig. 10.2

Answer : (a) 

[  Refractive index of the medium B relative to medium A  ]

5. A light ray enters from medium A to medium B as shown in Figure 10.2.

   

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) greater than unity .

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 Figure10.3.

 

Which of the following could be inside the box?

(a) A rectangular glass slab

(b) A convex lens

(c) A concave lens

(d) A prism

Answer :  (a) A rectangular glass slab .

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 10.4.

 

Which of the following could be inside the box?

(a) Concave lens

(b) Rectangular glass slab

(c) Prism

(d) Convex lens

Answer : (d) Convex lens .

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)  A convex lens has 4 dioptre power having a focal length 0.25 m .

[Here,

 We have,

]

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) less than one.

[ The purpose of a rear-view mirror in vehicles is to provide a wider field of view to the driver. To achieve this, the mirror typically forms an upright and diminished image of the objects behind the vehicle. As the image is diminished, the magnification (ratio of the image height to the object height) is less than one. So, the correct option is (a) less than one.]

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) 30 cm in front of the mirror .

[Given that the rays from the sun converge at a point 15 cm in front of a concave mirror, this point is the focal point (F) of the mirror. Therefore, the focal length  of the concave mirror is 15 cm.

 Here,

We have,  

Therefore, the object should be placed 30 cm in front of the mirror. ]

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) a convex mirror .

[ To see a full-length image of a distant tall building, the mirror should provide a wide field of view.

A convex mirror diverges light rays and allows a wider field of view compared to a concave or plane mirror. This makes a convex mirror suitable for seeing a full-length image of a distant object like a tall building. Therefore, the correct answer is: (b) a convex mirror . ]

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) very near to the focus of the reflector.

[ In torches, searchlights, and headlights of vehicles, the bulb is placed at the focus of the reflector. This positioning ensures that the light rays emitted from the bulb are reflected parallel to the principal axis, producing a strong and focused beam of light. Therefore, the correct answer is: (b) very near to the focus of the reflector . ]

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) all mirrors irrespective of their shape.

[ The laws of reflection state that the angle of incidence is equal to the angle of reflection, and these laws hold true for all types of mirrors, including plane mirrors, concave mirrors, and convex mirrors. So, the correct statement is that the laws of reflection are applicable to all mirrors, regardless of their shape.]

14. The path of a ray of light coming from air passing through a rectangular glass slab traced by four students are shown as A, B, C and D in Figure 10.5.

 

Which one of them is correct?

(a) A          (b) B        (c) C          (d) D

Answer : (b) B

15. You are given water, mustard oil, glycerine and kerosene. In which of these media a ray of light incident obliquely at same angle would bend the most?

(a) Kerosene       (b) Water       (c) Mustard oil     (d) Glycerine

Answer : (d) Glycerine .

[ The bending of a ray of light when it passes from one medium to another is determined by the refractive index of the media. The higher the refractive index, the more the light will bend. The refractive indices of the given substances are approximately:

Water: 1.33

Kerosene: 1.44

Mustard oil: 1.46

Glycerine: 1.47

Since glycerine has the highest refractive index among the given options, a ray of light incident obliquely at the same angle would bend the most in glycerine.

Therefore, the correct answer is: (d) Glycerine ]

16. Which of the following ray diagrams is correct for the ray of light incident on a concave mirror as shown in Figure 10.6?

    

(a) Fig. A       (b) Fig. B         (c) Fig. C       (d) Fig. D

Answer :  (d) Fig. D

[A ray parallel to the principal axis, after reflection in a concave mirror, passes through the principal focus.]

17. Which of the following ray diagrams is correct for the ray of light incident on a lens shown in Fig. 10.7?

       

(a) Fig. A.          (b) Fig. B.      (c) Fig. C.    (d) Fig. D.

Answer : (a) Fig. A

[A ray of light passing through a principal focus , after refraction from a convex lens, will emerge parallel to the principal axis .]

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) Concave, plane, and convex.

[ In this scenario, the child sees her head bigger, the middle portion of her body the same size, and her legs smaller.

This corresponds to the following order of mirror combinations from the top:

Concave Mirror: A concave mirror can create a magnified image when the object is within the focal length. This would make her head appear bigger.

Plane Mirror: A plane mirror produces an image that is the same size as the object, making the middle portion of her body appear the same size.

Convex Mirror: A convex mirror creates a diminished image, making her legs appear smaller.

Therefore, the correct answer is: (c) Concave, plane and convex . ]

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) Concave mirror, convex mirror, concave lens and convex lens

Answer : (d) Concave mirror, convex mirror, concave lens, and convex lens.

[ In the case of concave mirrors and convex lenses, when an object is placed at infinity, the image formed is highly diminished and point-sized. For convex mirrors, the image is also diminished and point-sized when the object is at infinity. Concave lenses can also produce a highly diminished image of an object at infinity. Therefore, all the given options (concave mirror, convex mirror, concave lens, and convex lens) can exhibit this behavior.]

Short Answer Questions

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 that 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) Object is placed between the device and its focus; the image formed is enlarged and behind it.

Device: Concave mirror

Explanation: When an object is placed between a concave mirror and its focus, the image formed is virtual, erect, and enlarged, and it appears behind the mirror.

(b) Object is placed between the focus and the device; the image formed is enlarged and on the same side as that of the object.

Device: Convex lens

Explanation: When an object is placed between the focus and a convex lens, the image formed is virtual, erect, and enlarged, and it appears on the same side as the object.

(c) Object is placed between infinity and the device; the image formed is diminished and between the focus and the optical center on the same side as that of the object.

Device: Concave lens

Explanation: When an object is placed at any position in front of a concave lens (between infinity and the lens), the image formed is virtual, erect, and diminished, and it appears between the focus and the optical center on the same side as the object.

(d) Object is placed between infinity and the device; the image formed is diminished and between the pole and focus, behind it.

Device: Convex mirror

Explanation: When an object is placed at any position in front of a convex mirror (between infinity and the mirror), the image formed is virtual, erect, and diminished, and it appears between the pole and the focus behind the mirror.

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: A light ray incident on a rectangular glass slab immersed in any medium emerges parallel to itself because the slab's parallel faces ensure that the angle of incidence equals the angle of emergence. Refraction occurs at both faces, causing the ray to bend inwards and then outwards symmetrically.

OE is the incident ray ; OO' is the refracted ray and O'H is the emergent ray .

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: No, the pencil will not appear to be bent to the same extent in kerosene or turpentine. The extent of bending depends on the refractive index of the liquid. Since kerosene (1.44) and turpentine (1.47) have higher refractive indices than water (1.33), the light bends more at the interface, making the pencil appear less bent in kerosene or turpentine compared to water.

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: The refractive index  of a medium is inversely related to the speed of light  in that medium. It measures how much the light slows down as it travels through the medium compared to its speed in a vacuum.

The refractive index of a medium is given by:  

Where ,  is the speed of light in a vacuum (  m/s), is the speed of light in the medium.

To find the refractive index of one medium relative to another, consider two media: Medium 1 with speed of light ​ and Medium 2 with speed of light  ​.

The refractive indices of these media are:    and

The relative refractive index  of Medium 1 with respect to Medium 2 is:

Thus, the refractive index of one medium relative to another can be expressed as the ratio of the speeds of light in those two media.

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: Here,   and

The refractive index of diamond with respect to glass is given by: 

Thus, the absolute refractive index of diamond is 2.4 .

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: Yes, the statement is correct. A convex lens can produce both magnified virtual and real images depending on the position of the object relative to the lens's focal length.

Magnified Virtual Image:

(i) To obtain a magnified virtual image, the object must be placed between the focal point () and the lens. Specifically, the object distance () should be less than the focal length ().

(ii) In this case, the image formed will be on the same side as the object, upright, and larger than the object. Mathematically, this means the object is within the focal length: 0 < < 20 cm.

Magnified Real Image:

(i) To obtain a magnified real image, the object should be placed between the focal point () and twice the focal length (). Specifically, the object distance (u) should be greater than the focal length () but less than twice the focal length ().

(ii) The image formed in this case will be on the opposite side of the lens, inverted, and larger than the object. Mathematically, this means the object distance should be within the range: 20 cm <  < 40 cm.

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: Sudha will need to move the screen away from the lens to focus on the building since the building is farther away than the window pane. The approximate focal length of the lens is 15 cm, as the sharp image of the window pane is formed at the focal length distance.

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  of a lens is inversely related to its focal length  by the formula:

          where  is the focal length in meters.

Given,  and 

We have ,

And

Therefore, you should use the lens with a focal length of 20 cm to obtain more convergent light.

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 : For the incident ray and reflected ray to always be parallel to each other in an arrangement of two plane mirrors, the angle between the two mirrors should be 90 degrees.

In this configuration, regardless of the angle of incidence, the angle of reflection will ensure that the reflected ray from one mirror becomes the incident ray on the other mirror, maintaining parallelism between the original incident ray and the final reflected ray.

     

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) from air into water :

    

OA - Incident ray (in air) ; OO' - refracted ray (in water) .

(ii) from water into air :

OO' - refracted ray (in water) ; O'B - emergent ray (in air) .

Long Answer Questions

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: 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

  

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: 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 :

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: The laws of refraction of light are :
(i) 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.
(ii) The ratio of sine of angle of incidence to the sine of angle of refraction is a constant, for the light of a given colour and for the given pair of media.

Consider ABCD as a rectangular glass slab. Draw a perpendicular NN′ to AB at O and another perpendicular MM′ to CDat O′ . The light ray at point O has entered from a rarer medium into a denser medium, that is, from air into glass. The light ray bends towards the normal. At O′, the light ray has entered from glass into air, that is, from a denser medium into a rarer medium. The light here bends away from the normal. Compare the angle of incidence with the angle of refraction at both refracting surfaces AB and CD.

In the figure, a ray EO is obliquely incident on surface AB, called the incident ray. OO′ is the refracted ray, and O′H is the emergent ray. We observe that the emergent ray is parallel to the direction of the incident ray. The extent of bending of the ray of light at the opposite parallel faces AB (air-glass interface) and CD (glass-air interface) of the rectangular glass slab is equal and opposite. This is why the ray emerges parallel to the incident ray. However, the light ray is shifted slightly sideways.

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: 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 :

      

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 : The image formation by a convex mirror when an object is placed (a) at infinity :

(b) at finite distance from the mirror

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 : Here,  and

We have ,  

Since the image is three times the size of the flame and formed on the opposite side of the lens (real and magnified), the lens must be a convex lens.

The image formed at 80 cm from the lens is real, inverted, and magnified.

36. Size of image of an object by a mirror having a focal length of 20 cm is observed to be reduced to rd of 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 ,   and

      

Using mirror formula , we have   

 

Therefore, the object distance of the convex mirror is 40 cm . The nature of the image is virtual and erect . 

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 lens and its power used by each of them?

Answer : The power of a lens is defined as the reciprocal of its focal length. It is represented by the letter . The power  of a lens of focal length  is given by :

Lens with Focal Length of 50 cm:

Convert focal length to meters:

We have,

Nature of the Lens: A positive focal length indicates a convex lens. It converges light.

Lens with Focal Length of -50 cm:

Convert focal length to meters:

 We have,

Nature of the Lens: A negative focal length indicates a concave lens. It diverges light.

38. A student focussed the image of a candle flame 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 lens = 50.0 cm

Position of the screen = 88.0 cm

(i) What is the focal length of the convex lens?

(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 (iii) as said above.

Answer: Given, Position of candle = 12.0 cm

Position of convex lens = 50.0 cm

Position of the screen = 88.0 cm

(i) Here  and

Using lens formula ,  

 

 

Therefore ,the focal length of the convex lens is 19 cm .

(ii) Here,  

Since the object distance is equal to the focal length, the image formed will be at infinity.            

(iii) Nature of the Image when Candle is Further Shifted :

If the new object distance is less than the focal length of the lens, the image will be virtual and erect.

(iv) The image formation of convex lens :