# 12th Physics chapter 9 Objective Questions

## Ray Optics and Optical Instruments Objective

1. For a total internal reflection, which of the following is correct?

(a) Light travels from rarer to denser medium.

(b) Light travels from denser to rarer medium.

(c) Light travels in air only.

(d) Light travels in water only.

Answer: (b) Light travels from denser to rarer medium.

1. Critical angle of glass is θ2and that of water is θ2. The critical angle for water and glass surface would be (μg = 3/2, μw = 4/3).

(a) less than θ2

(b) between θ1 and θ2

(c) greater than θ2

(d) less than θ1

Answer: (c) greater than θ2

1. Mirage is a phenomenon due to

(a) refraction of light

(b) reflection of light

(c) total internal reflection of light

(d) diffraction of light.

Answer: (c) total internal reflection of light

1. 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) become infinite

1. Which of the following forms a virtual and erect image for all positions of the object?

(a) Concave lens

(b) Concave mirror

(d) Convex mirror

(d) Both (a) and (c)

Answer: (d) Both (a) and (c)

1. Two lenses of focal lengths 20 cm and – 40 cm are held in contact. The image of an object at infinity will be formed by the combination at

(a) 10 cm

(b) 20 cm

(c) 40 cm

(d) infinity

Answer: (c) 40 cm

1. Two beams of red and violet color are made to pass separately through a prism (angle of the prism is 60°). In the position of minimum deviation, the angle of refraction will be

(a) 30° for both the colors

(b) greater for the violet color

(c) greater for the red color

(d) equal but not 30° for both the colors

Answer: (a) 30° for both the colors

1. Which of the following colours of white light deviated most when passes through a prism?

(a) Red light

(b) Violet light

(c) Yellow light

(d) Both (a) and (b)

Answer: (b) Violet light

1. An under-water swimmer cannot see very clearly even in absolutely clear water because of

(a) absorption of light in water

(b) scattering of light in water

(c) reduction of speed of light in water

(d) change in the focal length of eye lens

Answer: (d) change in the focal length of eye lens

1. An astronomical refractive telescope has an objective of focal length 20 m and an eyepiece of focal length 2 cm. Then

(a) the magnification is 1000

(b) the length of the telescope tube is 20.02 m

(c) the image formed of inverted

(d) all of these

Answer: (d) all of these

1. A rod of length 10 cm lies along the principal axis of a concave mirror of focal length 10 ’em 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) 5 cm

1. Optical fibers communication is based on which of the following phenomena?

(A) Total internal reflection

(B) Scattering

(C) Diffraction

(D) Refraction

Answer: (A) Total internal reflection

1. To get three images of a single object, one should place two plane mirrors at an angle of ……

(A) 60°

(B) 90°

(C) 120°

(D) 30°

1. In which mirror the image of an object always appears diminished and virtual?

(A) Plane

(B) Convex

(C) Concave

(D) None of these

1. With an increase in wavelength the refractive index:

(A) Decreases

(B) Increases

(C) Remains unaffected

(D) None of these

1. An object is placed between two plane mirrors inclined to each other at an angle of 60° The maximum number of images seen will be :

(A) 2

(B) 3

(C) 5

(D) 6

1. For which of the following pairs is the critical angle the smallest :

(A) Water-air

(B) Glass-water

(C) Glass-air

(D) Glass-Glass

1. When light enters glass from air, its wavelength :

(A) Increases

(B) Decreases

(C) Remains unchanged

(D) None of these

1. Two lenses of power -1.5 D and +2.5 D are placed in contact. The focal length of combination will be :

(A) 1 m

(B) 5 m

(C) 10 m

(D) 20 m

Answer: (A) 1 m

1. The length of an astronomical telescope is 16 cm and its magnifying power is 3. The focal length of the lenses will be :

(A) 4 cm, 12 cm

(B) 4 cm, 8 cm

(C) 4 cm,2 cm

(D) 8 cm, 4 cm

Answer: (A) 4 cm, 12 cm

1. A spherical mirror is immersed in water, its focal length will

(A) Increase

(B) Decrease

(C) Remain constant

(D) None of these

Answer: (C) Remain constant

1. A ray of light goes from glass (refractive index=5/3) to water (4/3). The critical angle will be :

(A) sin-1(1/2)

(B) sin-1(4/5)

(C) sin-1(5/6)

(D) None of these

1. A thin prism (refractive index = 3/2) in air is immersed in a liquid of refractive index 5/4. The ratio of angle of deviations in the two cases will be:

(A) 15/8

(B) 8/15

(C) 5/2

(D) 2/5

1. The refractive index of the material of an equilateral prism is √3. The angle of minimum deviation for the prism is :

(A) 30°

(B) 37°

(C) 45°

(D) 60°

1. Which colour of light has a shortest wavelength ?

(A) red

(B) yellow

(C) blue

(D) violet

1. For which colour of ray the refractive index of glass is largest ?

(A) red

(B) yellow

(C) blue

(d) violet

1. Which colour of light has longest wavelength ?

(A) red

(B) yellow

(C) blue

(D) violet

1. The component of white light that suffers the smallest deviation when passed through a glass prism is

(A) red

(B) violet

(C) green

(D) yellow

1. The power of a convex lens (refractive index 1.50) when immersed in water (refractive index 1.33)

(A) increases

(B) decreases

(C) remains unchanged

(D) None of these

1. As the wavelength is increased, the value of the refractive index of glass

(A) increases

(B) decreases

(C) remains the same

(D) None of these

1. If there are two lenses of powers 2 diopters and 3.5 diopters in contact, the power of the combination will be

(A) +1 D

(B) -1.5 D

(C) 2D

(D) -3.5 D

Answer: (B) -1.5 D

1. The ray of light does not deviate when it

(A) reflects from a plane mirror normally

(B) reflects from a plane mirror obliquely

(C) passes through a prism

(D) passes through a slab by refracting at parallel interfaces of slabe

Answer: (D) passes through a slab by refracting at parallel interfaces of slabe

1. The unit of power of a lens is

(A) lambda

(B) candela

(C) diopter

(D) watt

1. The power of a convex lens of focal length 10cm will be

(A) -10 D

(B) +10 D

(C) 95D

(D) +5 D

Answer: (B) +10 D

1. Mostly colour-blind eyes cannot distinguish between

(A) red and blue

(B) blue and green

(C) red and green

(D) red and yellow

Answer: (C) red and green

1. When a red flower’ is viewed through a green glass, it appears

(A) red

(B) green

(C) yellow

(D) black

1. If the distance between an object and its real image formed in a convex lens is 4f, the magnitude of magnification of the image is

(A) less than one

(B) equal to one

(C) more than one

(D) zero

Answer: (B) equal to one

1. A biconvex lens can form virtual image if the object lies

(A) between the lens and its focus

(B) at the focus

(C) between f and 2f

(D) at infinity

Answer: (A) between the lens and its focus

1. A parallel beam of rays strikes the plane surface of a thin planoconvex lens giving focal length f. If the lens be inverted, the focal length will be

(A) greater

(B) smaller

(C) of the same value

(D) zero

Answer: (C) of the same value

1. An object lies at 10 cm from a convex mirror of curvature radius 20 cm. The image distance is

(A) 20 cm

(B) 40 cm

(C) 5 cm

(D) infinite

Answer: (C) 5 cm

1. When concave lens of glass is immersed in water, it becomes

(A) less convergent

(B) convergent

(C) less divergent

(D) more divergent

Answer: (C) less divergent

1. If the focal length of a lens is 20cm, its power will be

(A) 5 diopter

(B) 20 diopter

(C) 0.5 diopter

(D) 0.05 diopter

Answer: (A) 5 diopter

1. If the power of a lens is +1D, its focal length will be

(A) 1 meter

(B) 0,5 meter

(C) 2 meter

(D) 1.5 meter

Answer: (A) 1 meter

1. When two converging lenses of same focal length fare placed in contact, then the focal length of the combination is

(A) f

(1) 2f

(C) f / 2

(D) 3f

Answer: (C) f / 2

1. A biconvex lens of air bubble in water will behave in the same way as a

(A) converging lens

(B) diverging lens

(C) glass slab

(D) concave mirror

Answer: (B) diverging lens

1. Two convex lenses are in contact. The equivalent lens is

(A) convex

(B) concave

(C) planoconcave

(D) cylindrical

1. The optical fibre works on the principle of

(A) Scattering

(B) Refraction

(C) Dispersion

(D) Total internal reflection

Answer: (D) Total internal reflection

1. A convex lens of focal length 40 cm is in contact with a convex lens of focal length 25 cm. The power of the combination is

(A) -1.5 D

(B) -6.5 D

(C) +6.5 D

(D) +5.6 D

Answer: (D) +5.6 D

1. A coloured image is formed when white rays pass through a thick lens because of

(A) reflection of light

(B) refraction of light

(C) chromatic aberration

(D) spherical aberration

Answer: (C) chromatic aberration

1. The equivalent focal length of the combination of two lenses with powers -15D and +5D is

(A) -10 cm

(B) -20 cm

(C) +10 cm

(D) None of these

Answer: (A) -10 cm

1. Which has larger difference in focal length ?

(A) in telescope

(B) compound microscope

(C) in both

(D) None of these

Answer: (A) in telescope