JEE Physics Wave Optics MCQs Set 21

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Chapter-wise Objective Questions: Wave Optics

View or download the dedicated Wave Optics MCQ resource below. Practicing these 50 objective questions regularly builds familiarity with standard exam patterns and helps secure higher marks in final Physics evaluations.

Multiple Choice Questions

Question 1: The phenomenon which is not explained by Huygens' construction of wavefront is
(a) reflection
(b) diffraction
(c) refraction
(d) origin of spectra
Show Answer & Explanation

Answer: (d) origin of spectra

Explanation:
1. Huygens' construction explains reflection, refraction and diffraction but not the origin of spectra.

Teacher's Note:
1. Recall the basic fact.

Question 2: The transverse nature of light was confirmed by the phenomenon of
(a) refraction of light
(b) diffraction of light
(c) dispersion of light
(d) polarization of light
Show Answer & Explanation

Answer: (d) polarization of light

Explanation:
1. Only transverse waves can be polarised, so polarisation proves the transverse nature of light.

Teacher's Note:
1. Recall the basic fact.

Question 3: Which of the following is correct for light diverging from a point source?
(a) The intensity decreases in inverse proportion to the square of the distance.
(b) The wavefront is parabolic.
(c) The intensity does not depend on the distance.
(d) None of these.
Show Answer & Explanation

Answer: (a) The intensity decreases in inverse proportion to the square of the distance.

Explanation:
1. Light from a point source spreads over a sphere of area 4πr².
2. So intensity falls as 1/r² and the wavefront is spherical.

Teacher's Note:
1. Recall the basic fact.

Question 4:
Assertion (A): No interference pattern is detected when two coherent sources are infinitely close to each other.
Reason (R): The fringe width is inversely proportional to the distance between the two sources.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. β = λD/d becomes infinitely large as d → 0.
2. So a single fringe would cover the whole screen and no pattern is seen.

Teacher's Note:
1. Think about what happens to β when d tends to zero.

Question 5: Unpolarised light of intensity 32 W m⁻² passes through three polarizers such that the transmission axis of the last polarizer is crossed with that of the first. The intensity of final emerging light is 3 W m⁻². The intensity of light transmitted by first polarizer will be
(a) 32 W m⁻²
(b) 16 W m⁻²
(c) 8 W m⁻²
(d) 4 W m⁻²
Show Answer & Explanation

Answer: (b) 16 W m⁻²

Explanation:
1. The first polarizer transmits half of the unpolarised light: 32/2 = 16 W m⁻².
2. (Check: with the middle polarizer at θ, 16cos²θ sin²θ = 4sin²2θ = 3 is possible.)

Teacher's Note:
1. Recall the basic fact.

Question 6: Which of the following statements is/are correct for diffraction? I. For diffraction through a single slit, the wavelength of wave must be comparable to the size of the slit. II. Diffraction is very common in sound waves but not so common in light waves. III. Diffraction is only observed in electromagnetic waves.
(a) Only I
(b) II and III
(c) I and II
(d) I, II and III
Show Answer & Explanation

Answer: (c) I and II

Explanation:
1. Diffraction needs a slit comparable to the wavelength, which is common for sound but not for light.
2. It is observed in all waves, so III is false.

Teacher's Note:
1. Recall the basic fact.

Question 7: A parallel beam of monochromatic unpolarised light is incident on a transparent dielectric plate of refractive index 1/√3. The reflected beam is completely polarised. Then the angle of incidence is
(a) 30°
(b) 60°
(c) 45°
(d) 75°
Show Answer & Explanation

Answer: (a) 30°

Explanation:
1. Brewster's law: tan i = 1/√3.
2. So i = 30°.

Teacher's Note:
1. Recall the basic fact.

Question 8: Optically active substances are those substances which
(a) produce polarised light
(b) produce double refraction
(c) rotate the plane of polarisation of polarised light
(d) convert a plane polarised light into circularly polarised light
Show Answer & Explanation

Answer: (c) rotate the plane of polarisation of polarised light

Explanation:
1. Optically active substances rotate the plane of polarisation, for example sugar solution.

Teacher's Note:
1. Recall the basic fact.

Question 9: Which of the following cannot produce two coherent sources?
(a) Lloyd's mirror
(b) Fresnel biprism
(c) Young's double slit
(d) Prism
Show Answer & Explanation

Answer: (d) Prism

Explanation:
1. Lloyd's mirror, biprism and double slit all derive two coherent sources from one source. A simple prism does not.

Teacher's Note:
1. Recall the basic fact.

Question 10: 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
Show Answer & Explanation

Answer: (a) a hyperbola

Explanation:
1. The points whose distances from two fixed points differ by a constant lie on a hyperbola.

Teacher's Note:
1. Recall the basic fact.

Question 11: 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
Show Answer & Explanation

Answer: (c) comparable to the wavelength of light used

Explanation:
1. Diffraction effects become noticeable when the size or separation is comparable to the wavelength.

Teacher's Note:
1. Recall the basic fact.

Question 12:
Assertion (A): In YDSE, if I₁ = 9I₀ and I₂ = 4I₀ then Imax/Imin = 25.
Reason (R): In YDSE Imax = (√I₁ + √I₂)² and Imin = (√I₁ − √I₂)².
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. Imax = (3 + 2)² I₀ = 25 I₀ and Imin = (3 − 2)² I₀ = I₀.
2. So the ratio is 25, using the formulae of the reason.

Teacher's Note:
1. Apply the formulae for Imax and Imin directly.

Question 13: A slit of width a is illuminated by red light of wavelength 6500 Å. If the first minimum falls at θ = 30°, the value of a is
(a) 6.5 × 10⁻⁴ mm
(b) 1.3 micron
(c) 3250 Å
(d) 2.6 × 10⁻⁴ cm
Show Answer & Explanation

Answer: (b) 1.3 micron

Explanation:
1. a sinθ = λ gives a = 6500 Å/sin30° = 13000 Å = 1.3 micron.

Teacher's Note:
1. Recall the basic fact.

Question 14: A beam of unpolarised light passes through a tourmaline crystal A and then through another such crystal B oriented so that its principal plane is parallel to A. The intensity of emergent light is I₀. Now B is rotated by 45° about the ray. The emergent light will have intensity
(a) \( I_0/2 \)
(b) \( I_0/\sqrt{2} \)
(c) \( I_0\sqrt{2} \)
(d) \( 2I_0 \)
Show Answer & Explanation

Answer: (a) \( I_0/2 \)

Explanation:
1. I = I₀cos²45° = I₀/2.

Teacher's Note:
1. Recall the basic fact.

Question 15: Light of wavelength 400 nm and 560 nm are incident simultaneously and normally on a double slit apparatus whose slit separation is 0.1 mm and screen distance is 1 m. The distance between areas of total darkness will be
(a) 4 mm
(b) 5.6 mm
(c) 14 mm
(d) 28 mm
Show Answer & Explanation

Answer: (d) 28 mm

Explanation:
1. Dark fringe positions: y = (2m + 1)λD/2d = (2m + 1) × 2 mm for 400 nm and (2n + 1) × 2.8 mm for 560 nm.
2. They coincide when 5(2m + 1) = 7(2n + 1), the first at 14 mm and the next at 42 mm.
3. Spacing of the totally dark areas = 28 mm.

Teacher's Note:
1. Recall the basic fact.

Question 16: 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) (2n − 1)π
(c) nπ
(d) (n + 1)π
Show Answer & Explanation

Answer: (b) (2n − 1)π

Explanation:
1. Destructive interference occurs for a phase difference of an odd multiple of π, that is (2n − 1)π.

Teacher's Note:
1. Recall the basic fact.

Question 17:
Assertion (A): A thin film such as a soap bubble or a thin layer of oil on water shows beautiful colours when illuminated by white light.
Reason (R): It happens due to the interference of light reflected from the upper and lower face of the thin film.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. The two reflected beams interfere.
2. Different wavelengths interfere constructively at different thicknesses, giving colours.

Teacher's Note:
1. Thin film colours are an interference effect.

Question 18: What is the Brewster angle for air to glass transition? (refractive index of glass = 1.5)
(a) \( \tan(1.5) \)
(b) \( \sin(1.5) \)
(c) \( \sin^{-1}(1.5) \)
(d) \( \tan^{-1}(1.5) \)
Show Answer & Explanation

Answer: (d) \( \tan^{-1}(1.5) \)

Explanation:
1. tan iB = μ = 1.5.
2. So iB = tan⁻¹(1.5).

Teacher's Note:
1. Recall the basic fact.

Question 19: In Young's double slit experiment using monochromatic light of wavelength λ, the path difference (in terms of an integer n) corresponding to any point having half the peak intensity is
(a) \( (2n+1)\frac{\lambda}{2} \)
(b) \( (2n+1)\frac{\lambda}{4} \)
(c) \( (2n+1)\frac{\lambda}{8} \)
(d) \( (2n+1)\frac{\lambda}{16} \)
Show Answer & Explanation

Answer: (b) \( (2n+1)\frac{\lambda}{4} \)

Explanation:
1. cos²(φ/2) = 1/2 gives φ/2 = π/4 (odd multiples), so φ = (2n + 1)π/2.
2. Path difference = φλ/2π = (2n + 1)λ/4.

Teacher's Note:
1. Recall the basic fact.

Question 20: 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 change with time
(c) increases linearly with time
(d) varies periodically with time
Show Answer & Explanation

Answer: (d) varies periodically with time

Explanation:
1. The electric vector of a wave oscillates periodically, perpendicular to the direction of propagation.

Teacher's Note:
1. Recall the basic fact.

Question 21: In Young's double slit experiment, the slits are 2 mm apart and are illuminated by photons of two wavelengths λ₁ = 12000 Å and λ₂ = 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) 6 mm
(b) 4 mm
(c) 3 mm
(d) 8 mm
Show Answer & Explanation

Answer: (a) 6 mm

Explanation:
1. β₁ = λ₁D/d = 1.2 mm and β₂ = 1.0 mm.
2. n₁β₁ = n₂β₂ gives 12n₁ = 10n₂, so n₁ = 5, n₂ = 6.
3. y = 5 × 1.2 = 6 mm.

Teacher's Note:
1. Recall the basic fact.

Question 22: Two coherent point sources S₁ and S₂ are placed on a line perpendicular to a screen, separated by a small distance d. The fringes obtained on the screen will be
(a) points
(b) straight lines
(c) semi-circles
(d) concentric circles
Show Answer & Explanation

Answer: (d) concentric circles

Explanation:
1. Points with the same path difference lie on a hyperboloid of revolution about the line S₁S₂.
2. Its intersection with the screen perpendicular to this line is a set of concentric circles.

Teacher's Note:
1. Recall the basic fact.

Question 23: 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
Show Answer & Explanation

Answer: (c) Reflected light is plane polarised but transmitted light is partially polarised

Explanation:
1. At Brewster's angle the reflected light is completely plane polarised.
2. The refracted light is only partially polarised.

Teacher's Note:
1. Recall the basic fact.

Question 24: In Young's double slit experiment, the fringes are displaced by a distance x when a glass plate of refractive index 1.5 is introduced in the path of one of the beams. When this plate is replaced by another plate of the same thickness, the shift of fringes is (3/2)x. The refractive index of the second plate is
(a) 1.75
(b) 1.50
(c) 1.25
(d) 1.00
Show Answer & Explanation

Answer: (a) 1.75

Explanation:
1. Shift is proportional to (μ − 1).
2. (μ' − 1)/(1.5 − 1) = 3/2, so μ' − 1 = 0.75 and μ' = 1.75.

Teacher's Note:
1. Recall the basic fact.

Question 25: In Young's double slit experiment, let β be the fringe width and let I₀ be the intensity at the central bright fringe. At a distance x from the central bright fringe, the intensity will be
(a) \( I_0\cos\left(\frac{x}{\beta}\right) \)
(b) \( I_0\cos^2\left(\frac{x}{\beta}\right) \)
(c) \( I_0\cos^2\left(\frac{\pi x}{\beta}\right) \)
(d) \( \frac{I_0}{4}\cos^2\left(\frac{\pi x}{\beta}\right) \)
Show Answer & Explanation

Answer: (c) \( I_0\cos^2\left(\frac{\pi x}{\beta}\right) \)

Explanation:
1. Phase difference φ = 2πx/β.
2. I = I₀cos²(φ/2) = I₀cos²(πx/β).

Teacher's Note:
1. Recall the basic fact.

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Multiple Choice Questions (MCQs) for JEE Physics Wave Optics

JEE Physics Wave Optics Objective Test Questions

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