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

Question: A beam of light from a source L is incident normally on a plane mirror fixed at a certain distance x from the source. The beam is reflected back as a spot on a scale placed just above the source L. When the mirror is rotated through a small angle q, the spot of the light is found to move through a distance y on the scale. The angle q is given by
(a) y/x
(b) x/2y
(c) x/y
(d) y/2x
Answer: d

Question: A beam of light composed of red and green ray is incident obliquely at a point on the face of rectangular glass slab. When coming out on the opposite parallel face, the red and green ray emerge from
(a) two points propagating in two different non parallel directions
(b) two points propagating in two different parallel directions
(c) one point propagating in two different directions (d) one point propagating in the same directions.
Answer: b

Question: An air bubble in a glass slab with refractive index 1.5 (near normal incidence) is 5 cm deep when viewed from one surface and 3 cm deep when viewed from the opposite face. The thickness (in cm) of the slab is
(a) 8
(b) 10
(c) 12
(d) 16.
Answer: c

Question: A rod of length 10 cm lies along the principal axis of a concave mirror of focal length 10 cm in such a way that its end closer to the pole is 20 cm away from the mirror. The length of the image is
(a) 10 cm
(b) 15 cm
(c) 2.5 cm
(d) 5 cm.
Answer: d

Question: Two plane mirrors are inclined at 70°. A ray incident on one mirror at angle, θ after reflection falls on second mirror and is reflected from there parallel to first mirror. The value of θ is
(a) 45°
(b) 30°
(c) 55°
(d) 50°
Answer: d

Question: An object is placed at a distance of 40 cm from a concave mirror of focal length 15 cm. If the object is displaced through a distance of 20 cm towards the mirror, the displacement of the image will be
(a) 30 cm away from the mirror
(b) 36 cm away from the mirror
(c) 30 cm towards the mirror
(d) 36 cm towards the mirror.
Answer: b

 Question: An equiconvex lens has power P. It is cut into two symmetrical halves by a plane containing the principal axis. The power of one part will be
(a) 0
(b) P/2
(c) P/4
(d) P
Answer: d

Question: A tall man of height 6 feet, want to see his full image. Then required minimum length of the mirror will be
(a) 12 feet
(b) 3 feet
(c) 6 feet
(d) any length 
Answer: b

Question: A ray of light travelling in a transparent medium of refractive index m, falls on a surface separating the medium from air at an angle of incidence of 45°. For which of the following value of m the ray can undergo total internal reflection?
(a) μ = 1.33
(b) μ = 1.40
(c) μ = 1.50
(d) μ = 1.25 
Answer: c

Question: A microscope is focussed on a mark on a piece of paper and then a slab of glass of thickness 3 cm and refractive index 1.5 is placed over the mark. How should the microscope be moved to get the mark in focus again?
(a) 2 cm upward
(b) 1 cm upward
(c) 4.5 cm downward
(d) 1 cm downward
Answer: b

Question: A plano convex lens is made of refractive index 1.6. The radius of curvature of the curved surface is 60 cm. The focal length of the lens is
(a) 200 cm
(b) 100 cm
(c) 50 cm
(d) 400 cm 
Answer: b

Question: A luminous object is placed at a distance of 30 cm from the convex lens of focal length 20 cm. On the other side of the lens, at what distance from the lens a convex mirror of radius of curvature 10 cm be placed in order to have an upright image of the object coincident with it ?
(a) 50 cm
(b) 30 cm
(c) 12 cm
(d) 60 cm
Answer: a

Question: Which of the following is not due to total internal reflection?
(a) Working of optical fibre
(b) Difference between apparent and real depth of a pond
(c) Mirage on hot summer days
(d) Brilliance of diamond
Answer: b

Question: Pick the wrong answer in the context with rainbow.
(a) Rainbow is a combined effect of dispersion, refraction and reflection of sunlight.
(b) When the light rays undergo two internal reflections in a water drop, a secondary rainbow is formed.
(c) The order of colours is reversed in the secondary rainbow.
(d) An observer can see a rainbow when his front is towards the sun.
Answer: d

Question: A boy is trying to start a fire by focusing sunlight on a piece of paper using an equiconvex lens of focal length 10 cm. The diameter of the sun is 1.39 × 10⁹ m and its mean distance from the earth is 1.5 × 10¹¹ m. What is the diameter of the sun’s image on the paper?
(a) 6.5 × 10^–5 m
(b) 12.4 × 10^–4 m
(c) 9.2 × 10^–4 m
(d) 6.5 × 10^–4 m
Answer: c

Question: In total internal reflection when the angle of incidence is equal to the critical angle for the pair of media in contact, what will be angle of refraction?
(a) 90°
(b) 180°
(c) 0°
(d) equal to angle of incidence 
Answer: a

Question: A ray is incident at an angle of incidence i on one surface of a small angle prism (with angle of prism A) and emerges normally from the opposite surface. If the refractive index of the material of the prism is m, then the angle of incidence is nearly equal to
(a) A/2μ
(b) 2A/μ
(c) μA
(d) μA/2 
Answer: c

Question: A convex lens is dipped in a liquid whose refractive index is equal to the refractive index of the lens. Then its focal length will
(a) become zero
(b) become infinite
(c) become small, but non-zero
(d) remain unchanged. 
Answer: b

Question: A converging beam of rays is incident on a diverging lens. Having passed through the lens the rays intersect at a point 15 cm from the lens on the opposite side. If the lens is removed, the point where the rays meet will move 5 cm closer to the lens. The focal length of the lens is
(a) 5 cm
(b) –10 cm
(c) 20 cm
(d) –30 cm 
Answer: d

Question: A astronomical telescope has objective and eyepiece of focal lengths 40 cm and 4 cm respectively. To view an object 200 cm away from the objective, the lenses must be separated by a distance
(a) 50.0 cm
(b) 54.0 cm
(c) 37.3 cm
(d) 46.0 cm.
Answer: b

Question: The angle of a prism is A. One of its refracting surfaces is silvered. Light rays falling at an angle of incidence 2A on the first surface returns back through the same path after suffering reflection at the silvered surface. The refractive index m, of the prism is
(a) 2sinA
(b) 2cosA
(c) 1/ 2 cos A
(d) tanA 
Answer: b

Question: A bulb is located on a wall. Its image is to be obtained on a parallel wall with the help of convex lens. The lens is placed at a distance d ahead of second wall, then required focal length will be
(a) only d/4
(b) only d/2
(c) more than d/4 but less than d/2
(d) less than d/4
Answer: b

Question: The blue colour of the sky is due to the phenomenon of
(a) scattering
(b) dispersion
(c) reflection
(d) refraction. 
Answer: a

Question: Which colour of the light has the longest wavelength?
(a) violet
(b) red
(c) blue
(d) green
Answer: b

Question: A convex lens and a concave lens, each having same focal length of 25 cm, are put in contact to form a combination of lenses. The power in diopters of the combination is
(a) zero
(b) 25
(c) 50
(d) infinite.
Answer: a 

Question: The refracting angle of a prism is A, and refractive index of the material of the prism is cot (A/2). The angle of minimum deviation is
(a) 90° – A
(b) 180° + 2A
(c) 180° – 3A
(d) 180° – 2A
Answer: d

Question: A biconvex lens has a radius of curvature of magnitude 20 cm. Which one of the following options describe best the image formed of an object of height 2 cm placed 30 cm from the lens?
(a) Virtual, upright, height = 1 cm
(b) Virtual, upright, height = 0.5 cm
(c) Real, inverted, height = 4 cm
(d) Real, inverted, height = 1 cm
Answer: c

Question: For a plano convex lens (m = 1.5) has radius of curvature 10 cm. It is silvered on its plane surface. Find focal length after silvering.
(a) 10 cm
(b) 20 cm
(c) 15 cm
(d) 25 cm
Answer: a

Question: The speed of light in media M₁ and M₂ are 1.5 × 10⁸ m/s and 2.0 × 10⁸ m/s respectively. A ray of light enters from medium M₁ to M₂ at an incidence angle i. If the ray suffers total internal reflection, the value of i is
(a) equal to sin−1(2/3)
(b) equal to or less than sin−1 (3/5)
(c) equal to or greater than sin-1(3/4)
(d) less than sin−1(2/3)
Answer: c

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

Question: Focal length of a convex lens of refractive index 1.5 is 2 cm. Focal length of lens when immersed in a liquid of refractive index of 1.25 will be
(a) 10 cm
(b) 2.5 cm
(c) 5 cm
(d) 7.5 cm
Answer: c

Question: The reddish appearance of the sun at sunrise and sunset is due to
(a) the scattering of light
(b) the polarisation of light
(c) the colour of the sun
(d) the colour of the sky.
Answer: a

Question: A thin prism of angle 15° made of glass of refractive index μ₁ = 1.5 is combined with another prism of glass of refractive index μ₂ = 1.75. The combination of the prisms produces dispersion without deviation. The angle of the second prism should be
(a) 5°
(b) 7°
(c) 10°
(d) 12°
Answer: c

Question: Rainbow is formed due to
(a) scattering and refraction
(b) internal reflection and dispersion
(c) reflection only
(d) diffraction and dispersion. 
Answer: b

Question: A person can see clearly objects only when they lie between 50 cm and 400 cm from his eyes. In order to increase the maximum distance of distinct vision to infinity, the type and power of the correcting lens, the person has to use, will be
(a) convex, +2.25 dioptre
(b) concave, –0.25 dioptre
(c) concave, –0.2 dioptre
(d) convex, +0.15 dioptre. 
Answer: b

Question: For the angle of minimum deviation of a prism to be equal to its refracting angle, the prism must be made of a material whose refractive index
(a) lies between √2 and 1
(b) lies between 2 and √2
(c) is less than 1
(d) is greater than 2 
Answer: b

Question: Exposure time of camera lens at f/2.8 setting is 1/200 second. The correct time of exposure at f/5.6 is
(a) 0.20 second
(b) 0.40 second
(c) 0.02 second
(d) 0.04 second.
Answer: c

Question: The magnifying power of a telescope is 9. When it is adjusted for parallel rays the distance between the objective and eyepiece is 20 cm. The focal length of lenses are
(a) 10 cm, 10 cm
(b) 15 cm, 5 cm
(c) 18 cm, 2 cm
(d) 11 cm, 9 cm. 
Answer: c

Question: An astronomical telescope of tenfold angular magnification has a length of 44 cm. The focal length of the objective is
(a) 44 cm
(b) 440 cm
(c) 4 cm
(d) 40 cm 
Answer: d

Question: A lens having focal length f and aperture of diameter d forms an image of intensity I. Aperture of diameter d/2 in central region of lens is covered by a black paper. Focal length of lens and intensity of image now will be respectively
(a) f and I/4
(b) 3f/4 and I/2
(c) f and 3I/4
(d) f/2 and I/2
Answer: c

Question: If f_V and f_R are the focal lengths of a convex lens for violet and red light respectively and F_V and F_R are the focal lengths of a concave lens for violet and red light respectively, then we must have
(a) f_V > f_R and F_V > F_R
(b) f_V < f_R and F_V > F_R
(c) f_V > f_R and F_V < F_R
(d) f_V < f_R and F_V < F_R
Answer: b

Question: A concave mirror of focal length f₁ is placed at a distance of d from a convex lens of focal length f₂. A beam of light coming from infinity and falling on this convex lens – concave mirror combination returns to infinity. The distance d must equal
(a) f₁ + f₂
(b) –f₁ + f₂
(c) 2f1 + f₂
(d) –2f₁ + f₂
Answer: c

Question: A bubble in glass slab (μ = 1.5) when viewed from one side appears at 5 cm and 2 cm from other side, then thickness of slab is
(a) 3.75 cm
(b) 3 cm
(c) 10.5 cm
(d) 2.5 cm
Answer: c

Question: If the refractive index of a material of equilateral prism is √3, then angle of minimum deviation of the prism is
(a) 60°
(b) 45°
(c) 30°
(d) 75°
Answer: a

Question: Two similar thin equi-convex lenses, of focal length f each, are kept coaxially in contact with each other such that the focal length of the combination is F₁. When the space between the two lenses is filled with glycerin (which has the same refractive index (m = 1.5) as that of glass) then the equivalent focal length is F₂. The ratio F₁ : F₂ will be
(a) 3 : 4
(b) 2 : 1
(c) 1 : 2
(d) 2 : 3
Answer: c

Question: Optical fibre are based on
(a) total internal reflection
(b) less scattering
(c) refraction
(d) less absorption coefficient. 
Answer: a

Question: Light enters at an angle of incidence in a transparent rod of refractive index π. For what value of the refractive index of the material of the rod, the light once entered into it will not leave it through its lateral face whatsoever be the value of angle of incidence?
(a) π = 1.1
(b) π = 1
(c) π > √2
(d) π = 1.3
Answer: c

Question: A thin prism having refracting angle 10° is made of glass of refractive index 1.42. This prism is combined with another thin prism of glass of refractive index 1.7. This combination produces dispersion without deviation. The refracting angle of second prism should be
(a) 6°
(b) 8°
(c) 10°
(d) 4°
Answer: a

Question: When a biconvex lens of glass having refractive index 1.47 is dipped in a liquid, it acts as a plane sheet of glass. This implies that the liquid must have refractive index
(a) equal to that of glass
(b) less than one
(c) greater than that of glass
(d) less than that of glass
Answer: a

Question: A ray of light is incident on a 60° prism at the minimum deviation position. The angle of refraction at the first face (i.e., incident face) of the prism is
(a) zero
(b) 30°
(c) 45°
(d) 60°
Answer: b