Physics MCQs for Class 12 with Answers Chapter 10 Wave Optics

Q.1. The locus of all particles in a medium, vibrating in the same phase is called
(a) wavelet
(b) fringe
(c) wave front
(d) None of these

Answer

Answer: (c)

Q.2. Which of the following is correct for light diverging from a point source?
(a) The intensity decreases in proportion for the distance squared.
(b) The wavefront is parabolic.
(c) The intensity at the wavelingth does depend of the distance.
(d) None of these.

Answer

Answer: (a)

Q.3. Wavefront is the locus of all points, where the particles of the medium vibrate with the same
(a) phase
(b) amplitude
(c) frequency
(d) period

Answer

Answer: (a) Wavefront is the locus of all points, where the particles of the medium vibrate with the same phase.

Q.4. The phenomena which is not explained by Huygen’s construction of wavefront
(a) reflection
(b) diffraction
(c) refraction
(d) origin of spectra

Answer

Answer: (d) The Huygen’s construction of wavefront does not explain the phenomena of origin of spectra.

Q.5. Huygen’s concept of secondary wave
(a) allows us to find the focal length of a thick lens
(b) is a geometrical method to find a wavefront
(c) is used to determine the velocity of light
(d) is used to explain polarisation

Answer

Answer: (b) Huyghen’s principle gives us a geometrical method of tracing a wavefront.

Q.6. A plane wave passes through a convex lens. The geometrical shape of the wavefront that emerges is
(a) plane
(b) diverging spherical
(c) converging spherical
(d) None of these

Answer

Answer: (c) Converging spherical

Q.7. Spherical wavefronts, emanating from a point source, strike a plane reflecting surface. What will happen to these wave fronts, immediately after reflection?
(a) They will remain spherical with the same curvature, both in magnitude and sign.
(b) They will become plane wave fronts.
(c) They will remain spherical, with the same curvature, but sign of curvature reversed.
(d) They will remain spherical, but with different curvature, both in magnitude and sign.

Answer

Answer: (c)

Q.8. When light suffers reflection at the interface between water and glass, the change of phase in the reflected wave is
(a) zero
(b) π
(c) π/2
(d) 2π

Answer

Answer: (b) Phase reversal occurs i.e. phase change = π takes place on reflection, because glass is much denser than water.

Q.9. Two plane wavefronts of light, one incident on a thin convex lens and another on the refracting face of a thin prism. After refraction at them, the emerging wavefronts respectively become
(a) plane wavefront and plane wavefront
(b) plane wavefront and spherical wavefront
(c) spherical wavefront and plane wavefront
(d) spherical wavefront and spherical wavefront
(e) elliptical wavefront and spherical wavefront

Answer

Answer: (c) Emerging wavefront will be spherical from convex lens and plane wavefront from the prism.

Q.10. When a film is illuminated by white light, its upper portion appears dark. Path difference between two reflected beams at the spot must be
(a) zero
(b) λ/2
(c) 2 λ/2
(d) π

Answer

Answer: (b)

Q.11. If two coherent sources are vibrating in phase then we have constructive interference at any point P whenever the path difference is

(a) (n+1/2)λ
(b) nλ/2
(c) (n-1/2)λ
(d) nλ

Answer

Answer: (d) When path difference = λ (n = 0, 1, 2…) the resultant intensity is 4I₀.

Q.12. If two sources are coherent, then the phase difference (􀁉) between the waves produced by them at any point
(a) will change with time and we will have stable positions of maxima and minima.
(b) will not change with time and we have unstable positions of maxima and minima.
(c) Positions of will not change with time and we will have stable positions of maxima and minima.
(d) will change with time and we will have unstable positions of maxima and minima.

Answer

Answer: (c)

Q.13. The device which produces highly coherent sources is
(a) Fresnel biprism
(b) Young’s double slit
(c) Laser
(d) Lloyd’s mirror

Answer

Answer: (c) Highly coherent sources are produced using laser.

Q.14. Which of the following, cannot produce two coherent sources?
(a) Lloyd’s mirror
(b) Fresnel biprism
(c) Young’s double slit
(d) Prism

Answer

Answer: (d) A prism cannot produce coherent sources.

Q.15. Coherence is a measure of
(a) capability of producing interference by wave
(b) waves being diffracted
(c) waves being reflected
(d) waves being refracted

Answer

Answer: (a) Coherence is a measure of capability of producing interference by waves.

Q.16. Two sources of light are said to be coherent, when they give light waves of same
(a) amplitude and phase
(b) wavelength and constant phase difference
(c) intensity and wavelength
(d) phase and speed

Answer

Answer: (b) For coherent sources λ is same and phase is also same or phase diff. is constant.

Q.17. Intensity of light depends on
(a) amplitude
(b) frequency
(c) wavelength
(d) velocity

Answer

Answer: (a) According to wave theory, intensity of light is directly proportional to square of amplitude.

Q.18. The colour of bright fringe nearest to central achromatic
fringe in the interference pattern with white light will be
(a) violet
(b) red
(c) green
(d) yellow

Answer

Answer: (a) As β=λD/d ⇒β α λ
As λ for violet is least, therefore, fringe nearest to central achromatic fringe will be violet.

Q.19. The correct formula for fringe visibility is
(a) V=(I_max-I_min)/(I_max+I_min)
(b) V=(I_max+I_min)/(I_max-I_min)
(c) V=I_max/I_min
(d) V=I_min/I_max

Answer

Answer: (a)

Q.20. Laser light is considered to be coherent because it consists of
(a) many wavelengths
(b) uncoordinated wavelengths
(c) coordinated waves of exactly the same wavelength
(d) divergent beam

Answer

Answer: (c) Laser light is coherent, because it consists of coordinated (parallel) waves of exactly same wavelength (i.e monochromatic wave).

Q.21. The interfering fringes formed by a thin oil film on water are seen in yellow light of sodium lamp. We find the fringes
(a) coloured
(b) black and white
(c) yellow and black
(d) coloured without yellow

Answer

Answer: (a) Bright fringes are yellow and dark fringes are black.

Q.22. In Young’s Double slit experiment, if the distance
between the slit and screen (D) is comparable with fringe
width (B), the fringe pattern on screen will
(a) strictly be a parabola
(b) strictly be a hyperbola
(c) be a elliptical
(d) be a straight line

Answer

Answer: (b) This time he saw a number of dark lines, regularly spaced; the first clear proof that light added to light can produce darkness.

Q.23. If Young’s double slit experiment is performed in water
keeping the rest of the set-up same, the fringes will
(a) increase in width
(b) decrease in width
(c) remain unchanged
(d) not be formed

Answer

Answer: (b) The wavelength of light in water (λ_w=λ_a/μ) is less than that in air. When the set-up is immersed in water, fringe width β(α λ) will decrease.

Q.24. In the Young’s Double slit experiment, when we place a converging lens after the slits and place the screen at the focus of the lens, it
(a) introduces an extra path difference in the parallel beam.
(b) introduces no path difference in the parallel beam.
(c) introduces an extra phase difference in the parallel beam.
(d) introduces an extra fringe in the diffraction pattern.

Answer

Answer: (b) Introducing a converging lens in the path of parallel beam does not introduce any extra path differences in a parallel beam. Rather it gives a more intense pattern on the screen.

Q.25. The fringe width for red colours as compared to that for violet colour is approximately
(a) 3 times
(b) 2 times
(c) 4 times
(d) 8 times

Answer

Answer: (b)As, β α λ
βred is roughly double of βviolet

Q.26. In Young’s double slit experiment, the minimum amplitude is obtained when the phase difference of super-imposing waves is (where n = 1, 2, 3, …)
(a) zero
(b) (2 n – 1) π
(c) n π
(d) (n + 1) π

Answer

Answer: (b) For minima, phase diff. = odd integral multiple of π = (2n-1)π

Q.27. The fringe width in a Young’s double slit experiment can
be increased if we decrease
(a) width of slits
(b) separation of slits
(c) wavelength of light used
(d) distance between slits and screen

Answer

Answer: (a) β=λD/d

Q.28. In Young’s double slit experiment, one slit is covered with
red filter and another slit is covered by green filter, then
interference pattern will be
(a) red
(b) green
(c) yellow
(d) invisible

Answer

Answer: (d) Interference pattern will be invisible, because red and green are complimentary colours.

Q.29. Instead of using two slits, if we use two separate identical sodium lamps in Young’s experiment, which of the following will occur?
(a) General illumination
(b) Widely separate interference
(c) Very bright maxima
(d) Very dark minima

Answer

Answer: (a) There will be general illumination as super imposing waves do not have constant phase difference.

Q.30. Which of the following is not essential for two sources of light in Young’s double slit experiment to produce a sustained interference?
(a) Equal wavelength
(b) Equal intensity
(c) Constant phase relationship
(d) Equal frequency

Answer

Answer: (b) Wavelength/frequency must be same and phase difference must be constant for producing sustained interference.

Q.31. In Young’s double slit experiment, the locus of the point P lying in a plane with a constant path difference between the two interfering waves is
(a) a hyperbola
(b) a straight line
(c) an ellipse
(d) a parabola

Answer

Answer: (a)

Q.32. If the width of the slit in single slit diffraction experiment is doubled, then the central maximum of diffraction pattern becomes
(a) broader and brighter
(b) sharper and brighter
(c) sharper and fainter
(d) broader adn fainter.

Answer

Answer: (b) Width of central maximum in diffraction pattern due to single slit = 2λD/d where λ is the wavelength, D is the distance between screen and slit and ‘a’ is the slit width. As the slit width a increases, width of central maximum becomes sharper or narrower. As same energy is distributed over a smaller area. therefore central maximum becomes brigther.

Q.33. A diffraction pattern is obtained by using beam of red. light what will happen, if red light is replaced by the blue light?
(a) Bands disappear.
(b) Bands become broader and farther apart.
(c) No change will take place.
(d) Diffraction bands become narrow and crowded together.

Answer

Answer: (d) When red light is replaced by blue light the diffraction bands become narrow and crowded.

Q.34. When monochromatic light is replaced by white light in Fresnel’s biprism arrangement, the central fringe is
(a) coloured
(b) white
(c) dark
(d) None of these

Answer

Answer: (b) At the centre, all colours meet in phase, hence central fringe is white.

Q.35. The condition for observing Fraunhoffer diffraction from a single slit is that the light wavefront incident on the slit should be
(a) spherical
(b) cylindrical
(c) plane
(d) elliptical

Answer

Answer: (c) Because both source & screen are effectively at infinite distance from the diffractive device

Q.36. The phenomenon of diffraction can be treated as interference phenomenon if the number of coherent sources is
(a) one
(b) two
(c) zero
(d) infinity

Answer

Answer: (d) Diffraction on a single slit is equivalent to interference of light from infinite number of coherent sources contained in the slit.

Q.37. The diffraction effects in a microscopic specimen become important when the separation between two points is
(a) much greater than the wavelength of light used.
(b) much less than the wavelength of light used.
(c) comparable to the wavelength of light used.
(d) independent of the wavelength of light used.

Answer

Answer: (c) When the wavelength of light used is comparable with the separation between two points, the image of the object will be a Φ diffraction pattern whose size will be,
θ=1.22λ/D, where λ = wavelength of light used; D= diameter of the objective
Two objects whose images are closer than this distance, will not be resolved.

Q.38. What is the Brewester’s angle for air to glass transition?( μ^a_g = 1.5)
(a) tan (1.5)
(b) sin (1.5)
(c) sin–1 (1.5)
(d) tan–1 (1.5)

Answer

Answer: (d)

Q.39. When ordinary light is made incident on a quarter wave plate, the emergent light is
(a) linearly polarised
(b) circulary polarised
(c) unpolarised
(d) elliptically polarised

Answer

Answer: (d)

Q.40. Transverse nature of light was confirmed by the phenomenon of
(a) refreaction of light
(b) diffraction of light
(c) dispersion of light
(d) polarization of light

Answer

Answer: (d) The phenomenon of polarization confirmed that light is a transverse wave.

Q.41. In the case of linearly polarized light, the magnitude of the electric field vector
(a) is parallel to the direction of propagation
(b) does not changes with time
(c) increases linearly with time
(d) varies periodically with time

Answer

Answer: (d) In the case of linearly polarised light the magnitued of th electric field vector varies periodically with time.

Q.42. Unpolarized light is incident on a plane glass surface The angle of incidence so that reflected and refracted rays are perpendicular to each other, them
(a) tan i_β = μ/2
(b) tan i_β = μ
(c) sin i_β = μ
(d) cos i_β = μ

Answer

Answer: (b) Brewster’s law is given by μ tan iβ

Q.43. Light waves can be polarised because they
(a) have high frequencies
(b) have short wavelength
(c) are transverse
(d) can be reflected

Answer

Answer: (c) Light waves can be polarized because they are transverse in nature.

Q.44. Light transmitted by nicol prism is
(a) unpolarised
(b) plane polarised
(c) circularly polarised
(d) elliptically polarised

Answer

Answer: (b) Light transmitted by nicol prism is plane polarised.

Q.45. Optically active substances are those substances which
(a) produces polarised light
(b) produces double refraction
(c) rotate the plane of polarisation of polarised light
(d) converts a plane polarised light into circularly polarised light.

Answer

Answer: (c) Such substances rotate the plane of polarised light.

Q.46. Polaroid glass is used in sun glasses because
(a) it reduces the light intensity to half on account of polarisation
(b) it is fashionable
(c) it has good colour
(d) it is cheaper.

Answer

Answer: (a) Polaroid glass polarises light reducing the light intensity to half its original value.

Q.47. In the propagation of light waves, the angle between the plane of vibration and plane of polarisaiton is
(a) 0º
(b) 90º
(c) 45º
(d) 80º

Answer

Answer: (b) Angle between plane of vibration and plane of polarisation is 90º.

Q.48. In the propagation of electromagnetic waves, the angle between the direction of propagation and plane of polarisation is
(a) 0º
(b) 45º
(c) 90º
(d) 180º

Answer

Answer: (a) Plane of vibration is ⊥ to direction of propagation and also ⊥ to plane of polarisation. Therefore, angle between plane of polarisation and direction of propagation is 0º.

Q.49. When unpolarised light is incident on a plane glass plate at Brewster’s angle, then which of the following statements is correct?
(a) Reflected and refracted rays are completely polarised with their planes of polarization parallel to each other
(b) Reflected and refracted rays are completely polarised with their planes of polarization perpendicular to each other
(c) Reflected light is plane polarised but transmitted light is partially polarised
(d) Reflected light is partially polarised but refracted light is plane polarised

Answer

Answer: (c) At Brewster’s angle, only the reflected light is plane polarised, but transmitted light is partially polarised.

Q.50. From Brewster’s law of polarisation, it follows that the anlge of polarisaiton depends upon
(a) the wavelength of light
(b) plane of polarisation’s orientation
(c) plane of vibration’s orientation
(d) None of these

Answer

Answer: (a) At Brewster’s angle, only the reflected light is plane polarised, but transmitted light is partially polarised.

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