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.

21. Why does a light ray incident on a rectangular glass slab immersed

in any medium emerges parallel to itself? Explain using a diagram.

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.

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?

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.

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not to be republished

84 EXEMPLAR PROBLEMS – SCIENCE

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?

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?

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?

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.

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.

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

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

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.

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not to be republished

LIGHT – REFLECTION AND REFRACTION 85

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

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

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?

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?

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?

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.