CBSE Class 12 Physics Wave Optics MCQs Set C

Question. In Young’s double slit experiment, if the separation between coherent sources is halved and the distance of the screen from the coherent sources is doubled, then the fringe width becomes
(a) double
(b) half
(c) four times
(d) one-fourth

Answer : C

Question. In a double slit experiment, when light of wavelength 400 nm was used, the angular width of the first minima formed on a screen placed 1 m away, was found to be 0.2°. What will be the angular width of the first minima, if the entire experimental apparatus is immersed in water? (mwater = 4/3)
(a) 0.1°
(b) 0.266°
(c) 0.15°
(d) 0.05°

Answer : C

Question. In a Young’ double slit experiment if there is no initial phase difference between the light from the two slits, a point on the screen corresponding to the fifth minimum has path difference.
(a) 5.λ/2
(b) 10.λ/2
(c) 9.λ/2
(d) 11.λ/2

Answer : C

Question. In Young’s double slit experiment the separation d between the slits is 2 mm, the wavelength l of the light used is 5896 Å and distance D between the screen and slits is 100 cm. It is found that the angular width of the fringes is 0.20°. To increase the fringe angular width to 0.21° (with same l and D) the separation between the slits needs to be changed to
(a) 1.8 mm
(b) 1.9 mm
(c) 2.1 mm
(d) 1.7 mm

Answer : B

Question. In Young’s double slit experiment, the slits are 2 mm apart and are illuminated by photons of two wavelengths l1 = 12000 Å and l2 = 10000 Å. At what minimum distance from the common central bright fringe on the screen 2 m from the slit will a bright fringe from one interference pattern coincide with a bright fringe from the other?
(a) 4 mm
(b) 3 mm
(c) 8 mm
(d) 6 mm

Answer : D

Question. In Young’s double slit experiment the distance between the slits and the screen is doubled. The separation between the slits is reduced to half. As a result the fringe width
(a) is halved
(b) becomes four times
(c) remains unchanged
(d) is doubled.

Answer : B

Question. Colours appear on a thin soap film and on soap bubbles due to the phenomenon of
(a) interference
(b) dispersion
(c) refraction
(d) diffraction.

Answer : A

Question. In a Fresnel biprism experiment, the two positions of lens give separation between the slits as 16 cm and 9 cm respectively. What is the actual distance of separation?
(a) 13 cm
(b) 14 cm
(c) 12.5 cm
(d) 12 cm

Answer : D

Question. The Brewsters angle ib for an interface should be
(a) 0° < ib < 30°
(b) 30° < ib < 45°
(c) 45° < ib < 90°
(d) ib = 90

Answer : C

Question. In Fresnel’s biprism expt., a mica sheet of refractive index 1.5 and thickness 6 × 10^–6 m is placed in the path of one of interfering beams as a result of which the central fringe gets shifted through 5 fringe widths. The wavelength of light used is
(a) 6000 Å
(b) 8000 Å
(c) 4000 Å
(d) 2000 Å

Answer : A

Question. A radar sends radiowaves of frequency v towards an aeroplane moving with velocity va. A change Dn is observed in the frequency of reflected waves which is higher than original frequency. The velocity of aeroplane is (va << c)
(a) cΔv/v
(b) 2cΔv/Δv
(c) cΔv/2v
(d) Δv/2cv

Answer : C

Question. In Young’s double slit experiment, we get 10 fringes in the field of view of monochromatic light of wavelength 4000Å. If we use monochromatic light of wavelength 5000Å, then the number of fringes obtained in the same field of view is
(a) 8
(b) 10
(c) 40
(d) 50

Answer : A

Question. A star, which is emitting radiation at a wavelength of 5000 Å, is approaching the earth with a velocity of 1.5 × 10⁴ m/s. The change in wavelength of the radiation as received on the earth is
(a) 25 Å
(b) 100 Å
(c) zero
(d) 2.5 Å

Answer : A

Question. Time taken by sunlight to pass through a window of thickness 4 mm whose refractive index is 3/2 is
(a) 2 × 10^–4 s
(b) 2 × 10⁸ s
(c) 2 × 10^–11 s
(d) 2 × 10¹¹ s 

Answer : C

Question. A beam of monochromatic light is refracted from vacuum into a medium of refractive index 1.5. The wavelength of refracted light will be
(a) depend on intensity of refracted light
(b) same
(c) smaller
(d) larger.

Answer : C

Question. Green light of wavelength 5460 Å is incident on an air-glass interface. If the refractive index of glass is 1.5, the wavelength of light in glass would be (c = 3 × 10⁸ m s^–1)
(a) 3640 Å
(b) 5460 Å
(c) 4861 Å
(d) none of the options

Answer : A

Question. In a double slit experiment, the two slits are 1 mm apart and the screen is placed 1 m away. A monochromatic light of wavelength 500 nm is used.
What will be the width of each slit for obtaining ten maxima of double slit within the central maxima of single slit pattern?
(a) 0.5 mm
(b) 0.02 mm
(c) 0.2 mm
(d) 0.1 mm

Answer : C

Question. At the first minimum adjacent to the central maximum of a single-slit diffraction pattern, the phase difference between the Huygen’s wavelet from the edge of the slit and the wavelet from the midpoint of the slit is
(a) Π radian
(b) Π /8 radian
(c) Π /4 radian
(d) Π/2 radian

Answer : A

Question. A telescope has an objective lens of 10 cm diameter and is situated at a distance of one kilometre from two objects. The minimum distance between these two objects, which can be resolved by the telescope, when the mean wavelength of light is 5000 Å, is of the order of
(a) 0.5 m
(b) 5 m
(c) 5 mm
(d) 5 cm

Answer : C

Question. Diameter of human eye lens is 2 mm. What will be the minimum distance between two points to resolve them, which are situated at a distance of 50 meter from eye? (The wavelength of light is 5000 Å.)
(a) 2.32 m
(b) 4.28 mm
(c) 1.25 cm
(d) 12.48 cm

Answer : C

Question. The ratio of resolving powers of an optical microscope for two wavelengths l1 = 4000 Å and l2 = 6000 Å is
(a) 9 : 4
(b) 3 : 2
(c) 16 : 81
(d) 8 : 27

Answer : B

Question. A linear aperture whose width is 0.02 cm is placed immediately in front of a lens of focal length 60 cm.
The aperture is illuminated normally by a parallel beam of wavelength 5 × 10–5 cm. The distance of the first dark band of the diffraction pattern from the centre of the screen is
(a) 0.10 cm
(b) 0.25 cm
(c) 0.20 cm
(d) 0.15 cm

Answer : D

Question. Ray optics is valid, when characteristic dimensions are
(a) much smaller than the wavelength of light
(b) of the same order as the wavelength of light
(c) of the order of one millimetre
(d) much larger than the wavelength of light.

Answer : D

Question. A parallel beam of monochromatic light of wavelength 5000 Å is incident normally on a single narrow slit of width 0.001 mm. The light is focussed by a convex lens on a screen placed in focal plane.
The first minimum will be formed for the angle of diffraction equal to
(a) 0°
(b) 15°
(c) 30°
(d) 50°

Answer : C

Question. The phenomenon by which stars recedes from each other is explained by
(a) black hole theory
(b) neutron star theory
(c) white dwarf
(d) red shift

Answer : D

Assertion (A) & Reason(R)

Question. Assertion : No interference pattern is detected when two coherent sources are infinitely close to each other.
Reason : The fringe width is inversely proportional to the distance between the two slits. Correct
Answer: a

Question. Assertion : When a light wave travels from a rarer to a denser medium, it loses speed. The reduction in speed imply a reduction in energy carried by the light wave.
Reason : The energy of a wave is proportional to velocity of wave. Correct
Answer: d

Question. Assertion: In Young’s experiment, the fringe width for dark fringes is different from that for white fringes.
Reason : In Young’s double slit experiment the fringes are performed with a source of white light, then only black and bright fringes are observed.
Answer: d

Question. Assertion : For best contrast between maxima and minima in the interference pattern of Young’s double slit experiment, the intensity of light emerging out of the two slits should be equal.
Reason : The intensity of interference pattern is proportional to square of amplitude. Correct
Answer: b

Question. Assertion : Interference pattern is made by using blue light instead of red light, the fringes becomes narrower.
Reason : In Young’s double slit experiment, fringe width is given by relation β = λD/d. Correct
Answer: a

Question. Assertion : When a tiny circular obstacle is placed in the path of light from some distance, a bright spot is seen at the centre of shadow of the obstacle.
Reason : Destructive interference occurs at the centre of the shadow. Correct
Answer: c

Question. Assertion : In Young’s double slit experiment if wavelength of incident monochromatic light is just doubled, number of bright fringe on the screen will increase.
Reason : Maximum number of bright fringe on the screen is directly proportional to the wavelength of light used.
Answer: d

Question. Assertion: Diffraction is common in sound but not common in light waves.
Reason : Wavelength of light is more than the wavelength of sound.
Answer: c

Question. Assertion : If complete YDSE (Young’s Double Slit Experiment) is dipped in the liquid from the air, then fringe width decreases.
Reason : Wavelength of light decreases, when we move from air to liquid.
Answer: a

Question. Assertion : In interference and diffraction, light energy is redistributed.
Reason : There is no gain or loss of energy, which is consistent with the principle of conservation of energy.
Answer: a