CBSE Class 12th Physics Wave Optics 100 MCQs

Physics – Wave Optics

Class 12 Physics (Chapter 10) • 100 Multiple Choice Questions

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Question 1

A wavefront is defined as the locus of all points in a medium having the same:

Correct Answer: Phase

Question 2

The direction of a ray of light is always:

Correct Answer: Perpendicular to the wavefront

Question 3

What is the shape of the wavefront for a point source of light at a finite distance?

Correct Answer: Spherical

Question 4

What is the shape of the wavefront for light coming from a very distant source (like a star)?

Correct Answer: Plane

Question 5

According to Huygens' Principle, every point on a wavefront acts as a source of secondary wavelets that spread out with the speed of light in that medium, and the new wavefront at any later time is given by:

Correct Answer: The forward envelope of these secondary wavelets

Question 6

Which phenomenon successfully proves the wave nature of light?

Correct Answer: Interference and Diffraction

Question 7

Constructive interference occurs at points where the phase difference between two interfering waves is an:

Correct Answer: Even multiple of π (2nπ)

Question 8

Destructive interference occurs at points where the phase difference between two interfering waves is an:

Correct Answer: Odd multiple of π (2n + 1)π

Question 9

In Young's Double Slit Experiment (YDSE), the fringe width (β) is given by:

Correct Answer: β = λ D / d

Question 10

In YDSE, if the whole apparatus is immersed in water (refractive index n > 1), what happens to the fringe width (β)?

Correct Answer: Decreases (becomes β / n)

Question 11

Coherent sources are those that emit light waves of:

Correct Answer: Same frequency, constant initial phase difference, and same wavelength

Question 12

Why are two independent light bulbs not coherent sources?

Correct Answer: Light emission is due to independent atomic transitions happening randomly in time and phase

Question 13

Diffraction is defined as:

Correct Answer: Bending of light around corners of an obstacle or aperture

Question 14

In a single-slit diffraction pattern, the condition for secondary minima is given by (where a is slit width):

Correct Answer: a sin θ = nλ (n = 1, 2, 3...)

Question 15

Polarization of light confirms that light waves are:

Correct Answer: Transverse in nature

Question 16

Brewster's Law states that when unpolarized light is incident at polarizing angle (Brewster's angle i_p), the reflected and refracted rays are:

Correct Answer: Mutually perpendicular (at 90° to each other)

Question 17

According to Brewster's Law, the refractive index (n) of a medium is related to Brewster's angle (i_p) as:

Correct Answer: n = tan(i_p)

Question 18

Malus's Law states that when completely plane-polarized light is incident on an analyzer, the transmitted intensity (I) varies as:

Correct Answer: I = I₀ cos² θ

Question 19

In Malus's Law (I = I₀ cos² θ), θ represents:

Correct Answer: Angle between the transmission axes of polarizer and analyzer

Question 20

What is the intensity of transmitted light when unpolarized light passes through a single ideal polarizer?

Correct Answer: Half of the incident intensity (I₀ / 2)

Question 21

What is the path difference corresponding to a phase difference of 2π radians?

Correct Answer: λ (Wavelength)

Question 22

In YDSE, if the intensity of two interfering waves is I₀ each, what is the maximum intensity at constructive interference?

Correct Answer: 4 I₀

Question 23

What is the minimum intensity in YDSE when two interfering waves have individual intensities I₀?

Correct Answer: Zero

Question 24

Which of the following describes corpuscular theory of light proposed by Newton?

Correct Answer: Light consists of tiny, massless, elastic particles called corpuscles

Question 25

According to Newton's corpuscular theory, speed of light in a denser medium (like water) compared to air is:

Correct Answer: Greater (which was later proven wrong by wave theory experiments)

Question 26

What happens to the fringe width in YDSE if the distance between the slits (d) is doubled?

Correct Answer: Halved

Question 27

What happens to the fringe width in YDSE if the distance between the slit and screen (D) is doubled?

Correct Answer: Doubled

Question 28

In a single-slit diffraction pattern, the width of the central maximum is:

Correct Answer: Twice the width of any secondary maximum (2λ D / a)

Question 29

What happens to diffraction fringes if monochromatic light in YDSE/diffraction is replaced by white light?

Correct Answer: Central white fringe with colored fringes on both sides

Question 30

The resolving power of a microscope can be increased by using:

Correct Answer: Light of shorter wavelength (like ultraviolet or electron beams)

Question 31

Unpolarized light has electric field vibrations in:

Correct Answer: All directions perpendicular to the direction of wave propagation

Question 32

Plane-polarized light has electric field vibrations confined to:

Correct Answer: A single plane perpendicular to the direction of propagation

Question 33

Which of the following waves CANNOT be polarized because they are longitudinal?

Correct Answer: Sound waves in air

Question 34

What is the path difference for the 3rd dark fringe in YDSE?

Correct Answer: 2.5 λ (or 5λ/2)

Question 35

What is the path difference for the 4th bright fringe in YDSE?

Correct Answer: 4λ

Question 36

Why does a soap bubble show beautiful colors when illuminated by white light?

Correct Answer: Interference of light reflected from the front and back surfaces of the thin film

Question 37

What is the condition for constructive interference in thin films reflected light (considering film thickness t and refractive index n)?

Correct Answer: 2 n t cos r = (n + 1/2)λ or 2nt = (2n+1)λ/2

Question 38

In a diffraction pattern, the width of secondary maxima compared to central maximum is:

Correct Answer: Half (λ D / a)

Question 39

Why are diffraction effects generally not observed with sound waves in everyday life objects?

Correct Answer: Wavelength of sound is comparable to everyday obstacle sizes

Question 40

What is the resolving power of a telescope whose objective has diameter D and light wavelength λ?

Correct Answer: D / (1.22 λ)

Question 41

What is the limit of resolution (minimum angular separation) for a telescope?

Correct Answer: 1.22 λ / D

Question 42

In Huygens' construction, each point on a wavefront is a source of:

Correct Answer: Secondary spherical wavelets

Question 43

Which law of optics can be proved using Huygens' Wave Theory?

Correct Answer: Both laws of reflection and laws of refraction (Snell's Law)

Question 44

What is the angle of incidence equal to in proof of reflection by Huygens' Principle?

Correct Answer: Angle of reflection (i = r)

Question 45

Interference phenomenon obeys which fundamental law of conservation?

Correct Answer: Conservation of energy (energy is redistributed, not destroyed)

Question 46

In YDSE, the 5th bright fringe is formed at a path difference of:

Correct Answer: 5λ

Question 47

In YDSE, the 4th dark fringe is formed at a path difference of:

Correct Answer: 3.5 λ (or 7λ/2)

Question 48

What is the angular width of central maximum in a single-slit diffraction pattern?

Correct Answer: 2λ / a

Question 49

What type of diffraction occurs when both source and screen are at finite distances from the obstacle?

Correct Answer: Fresnel diffraction

Question 50

What type of diffraction occurs when both source and screen are effectively at infinite distances (using lenses)?

Correct Answer: Fraunhofer diffraction

Question 51

What is the polarizing angle for water of refractive index 1.33?

Correct Answer: ≈ 53°

Question 52

Which device is used to produce or analyze plane-polarized light?

Correct Answer: Polaroid / Nicol prism

Question 53

What happens to the fringe pattern in YDSE if one of the slits is covered with a thin cellophane sheet of refractive index n and thickness t?

Correct Answer: The entire fringe pattern shifts towards the side of the covered slit

Question 54

What is the fringe shift (y) when a thin transparent sheet of thickness t and refractive index n is introduced in YDSE?

Correct Answer: Δy = (n - 1) t D / d

Question 55

Why are interference fringes not observed with two independent sodium lamps?

Correct Answer: Because the sources are incoherent and phase changes randomly every fraction of a nanosecond

Question 56

What is the angular position of the first minimum in single-slit diffraction of slit width a?

Correct Answer: sin θ = λ / a

Question 57

Which of the following phenomena proves that light travels in straight lines under normal circumstances?

Correct Answer: Rectilinear propagation (though wave nature explains diffraction around small edges)

Question 58

What is the shape of wavefront when a plane wave passes through a convex lens?

Correct Answer: Converging spherical wavefront

Question 59

What is the shape of wavefront when a plane wave passes through a concave lens?

Correct Answer: Diverging spherical wavefront

Question 60

In YDSE, if the distance between source and screen is doubled and slit separation is halved, the fringe width becomes:

Correct Answer: Four times

Question 61

What is the ratio of maximum to minimum intensity in an interference pattern where amplitudes of two interfering waves are a₁ and a₂?

Correct Answer: ((a₁ + a₂) / (a₁ - a₂))²

Question 62

Which scientist first observed the diffraction of light around obstacles?

Correct Answer: Francesco Grimaldi

Question 63

In a double-slit experiment, if one slit is covered, what happens to the interference pattern on the screen?

Correct Answer: Interference fringes disappear and a broad single-slit diffraction pattern appears

Question 64

What is the phase difference between two points on the same wavefront?

Correct Answer: Zero

Question 65

What is the phase difference between two consecutive wavefronts corresponding to path difference λ?

Correct Answer: 2π radians

Question 66

Which type of wavefront is produced by a linear source like a slit or fluorescent tube?

Correct Answer: Cylindrical wavefront

Question 67

In YDSE, if the wavelength of light used is doubled, the fringe width becomes:

Correct Answer: Doubled

Question 68

What is the central fringe in YDSE always?

Correct Answer: Bright fringe (maximum intensity)

Question 69

Which phenomenon explains why light bends around the edges of a shadow instead of casting a sharp geometric shadow?

Correct Answer: Diffraction

Question 70

What is the angle of polarization (Brewster's angle) for a medium of refractive index n = 1?

Correct Answer: 45° (since tan 45° = 1)

Question 71

When unpolarized light is incident at Brewster's angle on a glass surface, what is the polarization state of the reflected light?

Correct Answer: Completely plane-polarized

Question 72

In YDSE, if the distance between source and slits is decreased, what happens to the fringe pattern?

Correct Answer: Fringe width remains unaffected (since β depends on D, d, and λ)

Question 73

What is the resolving power of a microscope dependent on regarding numerical aperture (NA)?

Correct Answer: Directly proportional to NA (2 n sin θ / λ)

Question 74

Why can't light waves show interference if their polarizations are perpendicular to each other?

Correct Answer: Waves vibrating in mutually perpendicular planes do not interfere

Question 75

In YDSE, if white light is used, which color fringe is closest to the central white fringe?

Correct Answer: Violet

Question 76

Which color fringe is farthest from the central white fringe in YDSE using white light?

Correct Answer: Red

Question 77

What is the angular width of the central maximum in single-slit diffraction if slit width a equals wavelength λ?

Correct Answer: 2 radians (114.6°)

Question 78

What is the condition for secondary maxima in single-slit diffraction?

Correct Answer: a sin θ = (n + 1/2)λ (n = 1, 2, 3...)

Question 79

According to Huygens' wave theory, light waves are:

Correct Answer: Mechanical waves assumed to travel through an all-pervading hypothetical medium 'ether'

Question 80

What is the main difference between interference and diffraction patterns?

Correct Answer: Interference fringes are of equal width, while diffraction fringes vary in width and intensity

Question 81

What is the ratio of slit width to wavelength for distinct diffraction to be observed easily?

Correct Answer: Slit width should be comparable to wavelength (a ≈ λ)

Question 82

In YDSE, if the distance between the screen and slits is 1 m, slit separation is 1 mm, and wavelength is 500 nm, what is the fringe width?

Correct Answer: 0.5 mm

Question 83

What is the minimum path difference for destructive interference?

Correct Answer: λ / 2

Question 84

What is the minimum path difference for constructive interference?

Correct Answer: Zero

Question 85

Which of the following optical phenomena supports the particle (photon) nature of light rather than wave nature?

Correct Answer: Photoelectric effect

Question 86

What is the speed of light in a medium of refractive index 1.5?

Correct Answer: 2.0 × 10⁸ m/s

Question 87

When light reflects from a denser medium, what is the equivalent path difference introduced due to phase change of π?

Correct Answer: λ / 2

Question 88

In a double-slit experiment, if the slits are illuminated by white light, which fringes are colored?

Correct Answer: All fringes except the central fringe

Question 89

What is the condition for total destructive interference in terms of path difference (Δx)?

Correct Answer: Δx = (2n + 1) λ / 2

Question 90

What is the condition for total constructive interference in terms of path difference (Δx)?

Correct Answer: Δx = nλ (n = 0, 1, 2...)

Question 91

Which of the following phenomena cannot be explained by the wave theory of light?

Correct Answer: Photoelectric effect

Question 92

The wavefront originating from a point source at infinity is:

Correct Answer: Plane wavefront

Question 93

What is the intensity of light at minima in an interference pattern for ideal coherent sources of equal amplitude?

Correct Answer: Zero

Question 94

In a Young's double-slit experiment, the angular fringe width is given by:

Correct Answer: θ = λ / d

Question 95

Which of the following is an example of polarization by scattering?

Correct Answer: Polarization of sunlight scattered by air molecules (blue sky)

Question 96

What is the phase difference between two points having a path difference of λ / 4?

Correct Answer: π / 2 radians (90°)

Question 97

In a single-slit diffraction pattern, what is found at the very center of the pattern (θ = 0)?

Correct Answer: Central maximum (brightest and widest)

Question 98

What happens to the resolving power of a telescope if the diameter of its objective lens is increased?

Correct Answer: Increases

Question 99

Which of the following describes the Huygens' secondary wavelets concept correctly?

Correct Answer: Only forward wavelets are effective in contributing to the new wavefront (no backward wave)

Question 100

What is the total path difference at the position of the 1st secondary minimum in single-slit diffraction?

Correct Answer: λ