## LIGHT- REFLECTION AND REFRACTION Class 10 Exemplar

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Multiple Choice Questions:

Q-1: Which of the following can make a parallel 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

Solution: (a)

A ray passing through the principal focus of a concave mirror or convex lens, after reflection/refraction, will emerge parallel to the principal axis.

Q-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 of this mirror is

(a) 30 cm                        (b) 20 cm                          (c) 40 cm                            (d) 60 cm

Solution: (b)

Given object size, h = +10.0 mm                                                ( 1 cm = 10 mm)

= +1.0 cm

Image size h’ = 5.0 mm = 0.5 cm

Image distance, V=-30 cm                                                        (For real image)

Focal length,       f = ?

Q-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

Solution: (c)

A concave mirror can form an image enlarged, real and inverted than the actual object, beyond centre of curvature (C) when object is placed between the focus (F) and centre of curvature.

Q-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

Solution: (a)

Given, angle of incidence, i=600 , angle of refraction, r=450

Refractive index of the medium B relative to medium A,

Q-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

Solution: (a)

Since light rays in the medium B goes towards normal. So, it has greater refractive index and lesser velocity of light w.r.t. medium A. So, refractive index of medium B w.r.t. medium A is greater than unity.

Q-6: Beams of light are incident through the holes A and B and emerge out of the 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 lens                                                                         (d) A prism

Solution: (a)

Here, the emergent rays are parallel to the direction of the incident ray. Therefore, a rectangular glass slab could be inside the box as the extent of bending of light ray at the opposite parallel faces AB (air-glass interface) and CD (glass-air interface) of the rectangular glass slab are equal and opposite. This is why the ray emerges parallel to the incident ray.

Q-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

Solution: (d)

Since in the diagram all the parallel rays converge at a point. So, inside the box there will be a convex lens.

Q-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

Solution: (a) The power P of a lens of focal length f is given by

Q-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.

Solution: (a)

Te convex mirror forms virtual, erect an diminished image of the object. And rear view mirror also form some type of image. Therefore, magnification (m) produced by a rear view mirror fitted in vehicles is less than one i.e., m < 1.

Q-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

Solution: (b)

The rays from sun i.e., from infinity, are parallel to principal axis after reflection converge at a point, known as focus. Therefore, focal length (F) of concave mirror is 15 cm. And we know that, same size, real and inverted image is formed by concave mirror when object is placed at focus 2F or centre of curvature so to form same size image object will be placed at 15 X 2= 30 cm

Q-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

Solution: (b)

The convex mirror forms virtual, erect and diminished imaged of the objects. So, it can form full length image of a distant tall building.

Q-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

Solution: (b)

Concave mirrors are commonly used in torches, search lights and vehicles headlights to get powerful parallel beams of light.

Here the bulb is placed very near to the focus of the reflector as the incident rays, which passes, through the focus of concave mirror, after reflection become parallel to the principal axis of the mirror.

Q-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

Solution: (d)

The laws of reflection holds good for light reflected from any smooth surface i.e., all mirrors regardless of its shape.

Q-14: The path of 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.

Solution: (b)

In a rectangular glass slab, the emergent rays are parallel to the direction of the incident ray, because the lateral daviation of bending of the ray of light at the opposite parallel faces (air-glass interface) and (glass-air interface) of the rectangular glass slab are equal and opposite. This is why the ray emerges are parallel to the incident ray.

Q-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

Solution: (d)

The given material having their reflective index as kerosene is 1.44, water is 1.33, mustard oil is 1.46 and glycerine is 1.74. Thus, glycerine is most optically denser and hence have the highest refractive index. Therefore, ray of light bend most in glycerine.

Q-16: Which of the following ray diagram is correct for the ray of light incident on a concave mirror as shown in figure?

Solution: (d)

A ray parallel to the principal axis, after reflection will pass through the principal focus in case of a concave mirror.

Q-17: Which of the following ray diagram is correct for the ray of light incident on a lens shown in figure?

Solution: (a)

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

Q-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

Solution: (c)

Concave mirrors (of large focal length) can be used to see larger image of the head, the plane mirror for middle portion to see her body of the same size and convex mirror to see the diminished image of leg. Hence, the combinations for magic mirror from the top is concave mirror, plane mirror and convex mirror.

Q-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

Solution: (d)

The incident rays which comes from an object placed at infinity will be parallel and the rays parallel to the principal axis, after reflection/refraction by a concave mirror, convex mirror, a concave lens and convex lens, will pass or appear to pass through the principal focus. Hence, the image will be highly diminished and pint-sized.