Light

Exemplar Problems

MCQ Part 1

Question 1: Which of the following can make a parallel beam of light when light from a point source is incident on it?

  1. Concave mirror as well as convex lens
  2. Convex mirror as well as concave lens
  3. Two plane mirrors placed at 90° to each other
  4. Concave mirror as well as concave lens

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

Explanation: When point source of light is at the focus of concave mirror or convex lens, then a parallel beam of light is obtained after reflection (from concave mirror) and refraction (from convex lens).


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

  1. – 30 cm
  2. – 20 cm
  3. – 40 cm
  4. – 60 cm

Answer: (b) – 20 cm

Explanation: Given, height of object = 10 mm, height of image = 5 mm, image distance = -30 cm,

Magnification `=(h’)/h`

`=5/(10)=1/2=0.5`

Since, `m=v/u`

Or, `0.5=(-30text(cm))/u`

Or, `u=-(30)/(0.5)=-60` cm

Mirror formula `1/v+1/u=1/f`

Or, `-1/(60)-1/(30)=1/f`

Or, `(-1-2)/(60)=1/f`

Or, `1/f=-3/(60)=-1/(20)`

Or, `f=-20` cm

Question 3: Under which of the following conditions a concave mirror can form an image larger than the actual object?

  1. When the object is kept at a distance equal to its radius of curvature
  2. When object is kept at a distance less than its focal length
  3. When object is placed between the focus and centre of curvature
  4. When object is kept at a distance greater than its radius of curvature

Answer: (c) & (b)

Explanation: In both the cases image formed is larger than object. When object is between focus and pole of a concave mirror, then a larger, virtual and erect image is formed. When object is between C and F then image is formed beyond C. In this case, image is larger, real and inverted.


Question 4: Following 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

Refraction of light
  1. `(sqrt3)/(sqrt2)`
  2. `(sqrt2)/(sqrt3)`
  3. `1/(sqrt2)`
  4. `sqrt2`

Answer: (a) `(sqrt3)/(sqrt2)`

Explanation: As per Snell's Law,

`(n_B)/(n_A)=(text(sin)60°)/(text(sin)45°)`

`=(sqrt3)/2÷1/(sqrt2)`

`=(sqrt3)/2×sqrt2=(sqrt3)/(sqrt2)`

Question 5: A light ray enters from medium A to medium B as shown in following figure. The refractive index of medium B relative to A will be

Refraction of light
  1. greater than unity
  2. less than unity
  3. equal to unity
  4. zero

Answer: (a) Greater than unity

Explanation: This is similar to previous question, where ∠i > ∠r

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 following figure. Which of the following could be inside the box?

Refraction of light
  1. A rectangular glass slab
  2. A convex lens
  3. A concave lens
  4. A prism

Answer: (a) A rectangular glass slab

Explanation: Emerging rays are parallel to incident rays. This means that there is a rectangular glass slab inside the box.


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 following figure. Which of the following could be inside the box?

Refraction of light
  1. Concave lens
  2. Rectangular glass slab
  3. Prism
  4. Convex lens

Answer: (d) Convex lens

Explanation: Beams of light (parallel to each other) are converging after passing through the optical object. This can only be done by a converging lens.

Question 8: Which of the following statements is true?

  1. A convex lens has 4 dioptre power having a focal length 0.25 m
  2. A convex lens has –4 dioptre power having a focal length 0.25 m
  3. A concave lens has 4 dioptre power having a focal length 0.25 m
  4. 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

Explanation: Signs are same (positive) for both power and focal length in option a. Moreover, positive sign shows it is convex lens.

Question 9: Magnification produced by a rear view mirror fitted in vehicles

  1. is less than one
  2. is more than one
  3. is equal to one
  4. can be more than or less than one depending upon the position of the object in front of it

Answer: (a) is less than one

Explanation: A convex mirror is used as rearview mirror. A convex mirror always makes smaller image, because its field of view is largest among all types of mirrors.

Note: All figures on this page are taken from NCERT Exemplar Problems Class 10 Science Book


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