CBSE Class 12 Physics Ray Optics and Optical Instruments MCQs Set 05

Question: Mirage is a phenomenon due to
a) Diffraction of light
b) Refraction of light
c) Total internal reflection of light
d) None of these
Answer: c

Question: A biconvex lens has a focal length equal to
a) 2
b) 3
c) 3
d) 2
Answer: d

Question: A biconvex lens has a focal length 2/3 times the radius of curvature of either surface. The refractive index of the lens material is
a) 1.0
b) 1.5
c) 1.33
d) 1.75
Answer: d

Question: A screen is placed 90 cm away from an object. The image of the object on the screen is formed by a convex lens at two different positions separated by 20 cm. The focal length of the lens is
a) 10.7 cm
b) 21.4 cm
c) 5.5 cm
d) 42.8 cm
Answer: b

Question: A convergent beam of light passes through a diverging lens of focal length 0.2 m and comes to focus 0.3 m behind the lens. The position of the point at which the beam would converge in the absence of the lens is
a) 0.6 m
b) 0.12 m
c) 0.15 m
d) 0.3 m
Answer: b

Question: From a point source, light falls on a spherical glass surface (μ = 1.5, radius of curvature = 10 cm). The distance between point source and glass surface is 50 cm. The position of the image is
a) 150 cm
b) 25 cm
c) 50 cm
d) 100 cm
Answer: c

Question: The focal lengths of the objective and eyepiece of an astronomical telescope are 50 cm and 5 cm respectively. The length of the telescope when the image is formed at the least distance of distinct vision is
a) 55 cm
b) 45 cm
c) 325/6 cm
d) 275/6 cm
Answer: c

Question: Two lenses of power +10 D and –5 D are placed in contact. To obtain a virtual image of magnification 2, the object should be placed at
a) –10 cm
b) 10 cm
c) –5 m
d) 5 cm
Answer: a

Question: A convex lens of focal length 0.2 m made of glass (μ = 1.5) is immersed in water (μ = 1.33). The change in the focal length of the lens is
a) 5.8 cm
b) 0.58 m
c) 5.8 m
d) 0.58 cm
Answer: b

Question: A small telescope has an objective of focal length 144 cm and an eyepiece of focal length 6 cm. The separation between the objective and eyepiece is
a) 1.0 m
b) 0.75 m
c) 1.38 m
d) 1.5 m
Answer: d

Question: Two identical equiconvex glass lenses (μg = 3/2) of focal length f are kept in contact and the space between them is filled with water (μw = 4/3). The focal length of the combination is
a) f/2
b) f
c) 3f/4
d) 4f/3
Answer: c

Question: A ray of light passes through an equilateral prism (μ = 1.5) such that the angle of incidence equals the angle of emergence, and each is equal to three-fourths of the angle of the prism. The angle of deviation is
a) 60°
b) 45°
c) 120°
d) 30°
Answer: d

Question: A concave lens is placed in contact with a convex lens of focal length 25 cm. The combination forms a real image at 80 cm when the object is at 40 cm. The focal length of the concave lens is
a) +200 cm
b) –200 cm
c) –400 cm
d) +400 cm
Answer: c

Question: The critical angle for light going from medium (i) to medium (ii) is θ. If the speed of light in medium (i) is v, the speed of light in medium (ii) is
a) v cos θ
b) v / sin θ
c) v (1 – cos θ)
d) v / (1 – sin θ)
Answer: b

Question: Double convex lenses are made from glass of refractive index 1.55 with both surfaces having the same radius of curvature. If the focal length is 20 cm, the radius of curvature required is
a) 7 cm
b) 22 cm
c) 11 cm
d) 6 cm
Answer: b

Question:  Light travels in two media A and B with speeds 1.8 × 10⁸ m s^–1 and 2.4 × 10⁸ m s^–1 respectively. Then the critical angle between them is
(a) sin⁻¹[2/3]
(b) tan^{− 1}[3/4]
(c) tan⁻¹[2/3]
(d) sin⁻¹ [3/4] 
Answer: d

Question:  A point source of light is placed at a depth of h below the surface of water of refractive index m. A floating opaque disc is placed on the surface of water so that light from the source is not visible from the surface. The minimum diameter of the disc is
(a) 2h/ (μ² −1 ) ^1/2
(b) 2h(μ² – 1)^1/2
(c) h/2(μ² − 1)^1/2
(d) h(μ² – 1)^1/2   
Answer: a

Assertion & Reasoning Based MCQs

two statements are given-one labelled Assertion (A) and the other labelled Reason (R).
Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below.
(a) Both A and R are true and R is the correct explanation of A
(b) Both A and R are true but R is NOT the correct explanation of A
(c) A is true but R is false
(d) A is false and R is also false

Question:  Assertion (A) : Convergent lens property of converging remains same in all media.
Reason (R) : Property of lens whether the ray is diverging or converging is independent of the surrounding medium. 
Answer: D

Question:  Assertion (A) : Higher is the refractive index of a medium or denser the medium, lesser is the velocity of light in that medium.
Reason (R) : Refractive index is inversely proportional to velocity.
Answer: A

Question:  Assertion (A) : If optical density of a substance is more than that of water, then the mass density of substance can be less than water.
Reason (R) : Optical density and mass density are not related.
Answer: A

Question:  Assertion (A) : Endoscopy involves use of optical fibres to study internal organs.
Reason (R) : Optical fibres are based on phenomena of total internal reflection. 
Answer: A

Question:  Assertion (A) : Microscope magnifies the image.
Reason (R) : Angular magnification for image is more than object in microscope.   
Answer: A

Very Short Answer Type Questions 

Question:  A compound microscope is used because a realistic simple microscope does not have magnification.
Answer: A compound microscope is used because a realistic simple microscope does not have large magnification.

Question:  What is the angle of incidence for maximum deviation through a prism?
Answer: The deviation is maximum when angle is 90°.

Question:  A biconvex lens made of a transparent material of refractive index 1.25 is immersed in water of refractive index 1.33. Will the lens behave as a converging or a diverging lens? Give reason.
Answer: The lens will act as a diverging lens as the refractive index of water is greater than that of lens.

Case Based MCQs

Refraction Through Lens
A convex or converging lens is thicker at the centre than at the edges. It converges a parallel  beam of light on refraction through it. It has a real focus. Convex lens is of three types :
(i) Double convex lens (ii) Plano-convex lens (iii) Concavo-convex lens. Concave lens is thinner at the centre than at the edges. It diverges a parallel beam of light on refraction through it.
It has a virtual focus.

Question: Two thin lenses are in contact and the focal length of the combination is 80 cm. If the focal length of one lens is 20 cm, the focal length of the other lens is
a) –26.7 cm
b) 60 cm
c) 80 cm
d) 20 cm
Answer: a

Question: A point object is placed at a distance of 0.3 m from a convex lens of focal length 0.2 m. The lens is cut into two halves and each half is displaced by 0.0005 m as shown. The location of the image will be
a) 30 cm right of lens
b) 60 cm right of lens
c) 70 cm left of lens
d) 40 cm left of lens
Answer: b

Question: Lens used in a magnifying glass is
a) Concave lens
b) Convex lens
c) Both concave and convex lenses
d) Neither concave nor convex lens
Answer: b

Question: A spherical air bubble embedded in a piece of glass behaves for light like a
a) Converging lens
b) Diverging lens
c) Plano-converging lens
d) Plano-diverging lens
Answer: b

Question: The magnification produced by a convex lens is positive only when the object is placed
a) At its focus F
b) Between F and 2F
c) At 2F
d) Between F and the optical centre
Answer: d

Compound Microscope

A compound microscope is an optical instrument used for observing highly magnified images of tiny objects. Magnifying power of a compound microscope is defined as the ratio of the angle subtended at the eye by the final image to the angle subtended at the eye by the object, when both the final image and the object are situated at the least distance of distinct vision from the eye. It can be given that : m = me × mo, where me is magnification produced by eye lens and mo is magnification produced by objective lens.
Consider a compound microscope that consists of an objective lens of focal length 2.0 cm and an eyepiece of focal length 6.25 cm separated by a distance of 15 cm.

Question: How far from the objective should an object be placed in order to obtain the condition described in Q. No. 46?
a) 4.5 cm
b) 2.5 cm
c) 1.5 cm
d) 3.0 cm
Answer: b

Question: The object distance for the eyepiece, so that the final image is formed at the least distance of distinct vision, will be
a) 3.45 cm
b) –5 cm
c) –1.29 cm
d) 2.59 cm
Answer: b

Question: The magnifying power of a compound microscope increases when
a) The focal length of objective lens is increased and that of eyepiece is decreased
b) The focal length of eyepiece is increased and that of objective lens is decreased
c) Focal lengths of both objective and eyepiece are increased
d) Focal lengths of both objective and eyepiece are decreased
Answer: d

Question: The intermediate image formed by the objective of a compound microscope is
a) Real, inverted and magnified
b) Real, erect and magnified
c) Virtual, erect and magnified
d) Virtual, inverted and magnified
Answer: a