CBSE Class 12 Physics Optics Assignment Set B

Question. An object approaches a convergent lens from the left of the lens with a uniform speed 5 m/s and stops at the focus. The image
(a) moves away from the lens with an uniform speed 5 m/s
(b) moves away from the lens with an uniform acceleration
(c) moves away from the lens with a non-uniform acceleration
(d) moves towards the lens with a non-uniform acceleration

Answer: C

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) 12 cm
(b) 30 cm
(c) 50 cm
(d) 60 cm

Answer: C

Question. A passenger in an aeroplane shall
(a) never see a rainbow
(b) may see a primary and a secondary rainbow as concentric circles
(c) may see a primary and a secondary rainbow as concentric arcs
(d) shall never see a secondary rainbow

Answer: B

Question. There are certain material developed in laboratories which have a negative refractive index figure. A ray incident from air (Medium 1) into such a medium (Medium 2) shall follow a path given by 

Answer: A

Question. An air bubble in glass slab (μ = 1.5) from one side is 6 cm and from other side is 4 cm. The thickness of glass slab is
(a) 10 cm
(b) 6.67 cm
(c) 15 cm
(d) None of these

Answer: C

Question. A concave mirror forms the image of an object on a screen. If the lower half of the mirror is covered with an opaque card, the effect would be to make the
(a) image less bright.
(b) lower half of the image disappear.
(c) upper half of the image disappear.
(d) image blurred.

Answer: A

Question. The direction of ray of light incident on a concave mirror is shown by PQ while directions in which the ray would travel after reflection is shown by four rays marked 1, 2, 3 and 4 (figure). Which of the four rays correctly shows the direction of reflected ray? 

(a) 1
(b) 2
(c) 3
(d) 4

Answer: B

Question. An air bubble in a glass slab (μ = 1.5) is 5 cm deep when viewed from one face and 2 cm deep when viewed from the opposite face. The thickness of the slab is
(a) 7.5 cm
(b) 10.5 cm
(c) 7 cm
(d) 10 cm

Answer: B

Question. A glass slab of thickness 4 cm contains the same number of waves as 5 cm of water when both are traversed by the same monochromatic light. If the refractive index of water is 4/3, what is that of glass?
(a) 5/3
(b) 5/4
(c) 16/15
(d) 1.5

Answer: A

Question. A light ray falls on a rectangular glass slab as shown. The index of refraction of the glass, if total internal reflection is to occur at the vertical face, is
(a) √3/ 2
(b) (√3 + 1)/2
(c) (√2 + 1)/2
(d) √5 / 2

Answer: A

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

Answer: B

Question. If the focal length of objective lens is increased then magnifying power of :
(a) microscope will increase but that of telescope decrease.
(b) microscope and telescope both will increase.
(c) microscope and telescope both will decrease
(d) microscope will decrease but that of telescope increase.

Answer: D

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 μ, of the prism is : 
(a) 2 sin A
(b) 2 cos A
(c) 1/2 cos A
(d) tan A

Answer: B

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) 180° – 2A
(b) 90° – A
(c) 180° + 2A
(d) 180° – 3A

Answer: A

Question. A plano convex lens fits exactly into a plano concave lens. Their plane surfaces are parallel to each other. If lenses are made of different materials of refractive indices μ₁ and μ₂ and R is the radius of curvature of the curved surface of the lenses, then the focal length of the combination is 

Answer: B

Question. Light enters at an angle of incidence in a transparent rod of refractive index n. 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) n > √2
(b) n = 1
(c) n = 1.1
(d) n = 1.3

Answer: A

Question. The angle of incidence for a ray of light at a refracting surface of a prism is 45° . The angle of prism is 60°. If the ray suffers minimum deviation through the prism, the angle of minimum deviation and refractive index of the material of the prism respectively, are : 
(a) 45° , 1/√2
(b) 30°, √2
(c) 45°, √2 
(d) 30°, 1/√2 

Answer: B

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) 37.3 cm
(b) 46.0 cm
(c) 50.0 cm
(d) 54.0 cm

Answer: D

Question. An equiconvex lens is cut into two halves along (i) XOX' and (ii) YOY' as shown in the figure. Let f, f', f'' be the focal lengths of the complete lens, of each half in case (i), and of each half in case (ii), respectively 

Choose the correct statement from the following
(a) f ' = 2f, f '' = 2f
(b) f ' = f, f '' = 2f
(c) f ' = 2f, f '' = f
(d) f ' = f, f '' = f

Answer: B

Question. The optical density of turpentine is higher than tnat of water while its mass density is lower. Figure shows a layer of turpentine floating over water in a container. For which one of the four rays incident on turpentine in figure, the path shown is correct? 
(a) 1
(b) 2
(c) 3
(d) 4

Answer: B

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

Answer: A

Question. Match the corresponding entries of column-1 with column-2 (Where m is the magnification produced by the mirror):
Column-1         Column-2 
(A) m = –2        (a) Convex mirror
(B) m = -1/2     (b) Concave mirror
(C) m = +2       (c) Real image
(D) m = + 1/2   (d) Virtual image
(a) A → b and c, B → b and c, C → b and d, D → a and d.
(b) A → a and c, B → a and d, C → a and b, D → c and d
(c) A → a and d, B → b and c, C → b and d, D → b and c
(d) A → c and d, B → b and d, C → b and c, D → a and d

Answer: A

Directions for : Each question contains STATEMENT-1 and STATEMENT-2. Choose the correct answer (ONLY ONE option is correct ) from the following-
(a) Statement -1 is false, Statement-2 is true
(b) Statement -1 is true, Statement-2 is true; Statement -2 is a correct explanation for Statement-1
(c) Statement -1 is true, Statement-2 is true; Statement -2 is not a correct explanation for Statement-1
(d) Statement -1 is true, Statement-2 is false

Question. Statement 1: Optical fibres are used to transmit light without any appreciable loss in its intensity over distance of several kilometers.
Statement 2 : Optical fibres are very thick and all the light is passed through it without any loss.

Answer: D

Question. Statement 1: Two convex lenses joined together cannot produce an achromatic combination.
Statement 2 : The condition for achromatism is ω₁/f₁ + ω₂/f₂ = 0 where symbols have their usual meaning. 

Answer: B

Question. Statement 1 : If P₁ and P₂ be the powers of two thin lenses located coaxially in a medium of refractive index μ at a distance d, then the power P of the combination is
P = P₁ + P₂ – P₁P₂d/μ
Statement 2 : Because for above given system equivalent focal length is given by F = f₁f₂/f₁ + f₂ - d/μ and P = 1/F .

Answer: B

Question. Statement 1: Critical angle is minimum for violet colour. 
Statement 2 : Because critical angle θ_c = sin^-1(1/μ) and μ∞ (1)/λ .

Answer: B