Chapter 9: 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.

[ A point source placed at the focus of a concave mirror or convex lens produces a parallel beam after reflection or refraction.]

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.

[ When the object is placed between the focus (F) and the center of curvature (C), the image is real, inverted and magnified (larger than the object).]

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)

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:  (b) less than unity

[ The light ray bends away from the normal in medium B, showing it is optically rarer. Therefore, the refractive index of B relative to A is less than one. ]

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 .

[ The incident light rays enter the box and emerge parallel to their original path, which is a key property of 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 .

[ Inside the box is a convex lens. It converges the parallel incident rays to a focus point, causing them to cross over and emerge inverted on the other side.]

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.

[ A rear-view mirror is a convex mirror. It always forms a virtual, erect, and diminished image of the objects behind the vehicle. Therefore, the magnification produced by a rear-view mirror is always 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 .

[ 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

[ A convex mirror always forms a diminished (smaller), virtual and erect image. Because the image is smaller, a large distant object like a tall building can be seen completely in the 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 vehicle headlights, the bulb is placed near the focus of a concave reflector. The reflected rays then become almost parallel, producing a strong beam of light that can travel a long distance.]

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 apply to every type of mirror, whether plane concave, or convex. In all cases, the angle of incidence is equal to the angle of reflection, and the incident ray, reflected ray and normal lie in the same plane.]

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

[ Light entering glass bends towards the normal . Light exiting into air bends away from the normal, making the emergent ray parallel to the incident ray. ]

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 .

[ A ray of light bends more in a medium having a higher refractive index. Among water, kerosene, mustard oil and glycerine, glycerine has the highest refractive index. Therefore, light bends the most in 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

[ A concave mirror makes the head appear bigger, a plane mirror shows the middle part in the same size and a convex mirror makes the legs appear smaller.]

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

[ For an object placed at infinity, these mirrors and lenses form a highly diminished, almost point-sized image at their principal focus.]

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) Concave mirror (object between pole and focus) → Virtual, erect, enlarged image, behind the device

(b) Convex lens (object between optical centre and focus) → Virtual, erect, enlarged image, same side as object .

(c) Concave lens (object anywhere) → Virtual, erect, diminished image, same side as object, between F and optical centre

(d) Convex mirror (object anywhere) → Virtual, erect, diminished image, behind device, between pole and focus .

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: When a light ray enters a rectangular glass slab, it bends towards the normal due to change in medium (refraction). Inside the slab, it travels straight and then bends away from the normal while emerging. Since the opposite faces of the slab are parallel, the angle of emergence becomes equal to the angle of incidence. Thus, the emergent ray is parallel to the incident ray, with a slight lateral displacement.

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.

If c is the speed of light in vacuum and is the speed in a medium, then the absolute refractive index is:

If the speed of light in medium 1 is and in medium 2 is ​, then the refractive index are and .

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

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 form both magnified virtual and real images depending on the object position.

(i) For a magnified virtual image, the object should be placed between the optical centre and focus (less than 20 cm).

(ii) For a magnified real image, the object should be placed between the focus and twice the focal length (between 20 cm and 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 move the screen towards the lens because the building is farther than the window pane, so the image distance decreases. The lens forms a sharp image of the window pane at 15 cm, meaning the image distance is 15 cm when the object is at a large distance (window). Therefore, the approximate focal length of the lens is 15 cm.

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: When two plane mirrors are placed parallel to each other, the incident ray and the final reflected ray will always remain parallel, whatever be the angle of incidence.

The diagram of two plane mirrors :

    

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 to water (AO to OO'): The ray enters a denser medium and bends towards the normal. The refracted ray makes a smaller angle with the normal.

(ii) From water to air (OO' to O'B): The ray enters a rarer medium and bends away from the normal. The refracted ray makes a larger angle with the normal.

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: (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 at centre of curvature of the mirror :

(d) The image formation by a concave mirror when an object is placed a little beyond centre of curvature of the mirror :

(e) The image formation by a concave mirror when an object is placed 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:  (a) The image formation by a convex lens when an object is placed between optical centre and focus of the lens :

(b) The image formation by a convex lens when an object is placed between focus and twice the focal length of the lens:

(c) The image formation by a convex lens when an object is placed at twice the focal length of the lens:

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

(e) The image formation by a convex lens when an object is placed 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: Laws of refraction: (i) The incident ray, the refracted ray, and the normal at the point of incidence all lie in the same plane.

(ii) For a given pair of media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant (Snell’s law):  .

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: (a) The image formed by a concave lens when an object is placed at its focus :

            

(b) The image formed by a concave lens, when an object is placed between the focus and twice the focal length:

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

(b) The image formation by a convex mirror when an object is placed 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 , 

So, the candle should be placed at a distance of about 26.7 cm from the lens.

Since the image is obtained on a screen, it is real and inverted. As the image is larger than the object, it is a magnified real image formed by a convex lens.

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 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  

 and

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: Here,

We have, 

This is a convex lens.

Lens with Focal Length of - 50 cm: Here, 

 We have,

This is a concave lens.

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 ray diagram of convex lens :