CBSE Class 12 Physics Wave Optics PYQs Analysis (2021-2025)

Based on detailed analysis of last 5 years' papers. Perfect for 2026 Boards prep!

Key Trends

Weightage Breakdown: 1-3 MCQs/assertion-reason (1 mark), 1-2 short answers (2 marks), 1-2 numericals/derivations on interference/diffraction/polarisation (4-6 marks).
Repeated Topics: Huygens principle & wavefronts, interference (Young’s double slit experiment, fringe width, path difference), diffraction (single slit, angular width), polarisation (plane, circular, Malus law, Brewster’s law), coherent sources, intensity distribution (I ∝ cos²(δ/2)), resolving power of microscope/telescope.
Difficulty: Medium; YDSE fringe width & single slit diffraction numericals are most repeated & scoring.
Changes: More numericals on fringe shift due to thin film/sheet & polarisation angle in 2023–2025.

Most Important and Repeated Questions

Question Example	Type/Marks	Years Repeated	Notes
Explain Huygens principle. Using it, derive laws of reflection and refraction.	Derivation (3-4 marks)	2021, 2022, 2023, 2024, 2025	Repeated 5x; Each point on wavefront acts as secondary source → envelope gives new wavefront → prove i = r & Snell’s law.
In Young’s double slit experiment, derive expression for fringe width β = λD/d. What happens when screen is moved away or slit separation changes?	Derivation/Numerical (4 marks)	2021 Term 2, 2022, 2023, 2024	Repeated 4x; Path difference δ = d sinθ ≈ d x/D → constructive: δ = nλ → x = n λ D/d → β = λ D/d.
Explain single slit diffraction. Derive angular width of central maximum θ = 2λ/a or intensity distribution I = I₀ (sinβ/β)².	Derivation (3-4 marks)	2022, 2023, 2024, 2025	Repeated 4x; Path difference from edges → minima at sinθ = nλ/a → central max width 2λ/a.
Assertion: In YDSE, central fringe is always bright. Reason: Path difference at centre is zero → constructive interference.	Assertion-Reason (1 mark)	2023, 2024, 2025	Repeated 3x; Both true, reason explains.
A thin transparent sheet is introduced in front of one slit in YDSE. Find shift in fringe pattern (path difference introduced = (μ-1)t).	Numerical (3 marks)	2021 Term 1, 2022, 2023, 2025	Repeated 4x; Shift β' = (μ-1)t D/d = [(μ-1)t / λ] × β towards sheet side.
State Brewster’s law. Find angle of polarisation i_p = tan⁻¹(μ). Explain polarisation by reflection.	Short Answer (2-3 marks)	2022, 2023, 2024	Repeated 3x; At i_p, reflected light fully polarised (parallel to plane of incidence); tan i_p = μ.
MCQ: Fringe width in YDSE is proportional to: (a) λ (b) D (c) 1/d (d) all of these	MCQ (1 mark)	2021 Term 1, 2023, 2024	Repeated 3x; Answer (d) all (β = λ D/d).
Explain resolving power of microscope & telescope. How does it depend on wavelength & aperture?	Short Answer (3 marks)	2023, 2025	Repeated 2x; Microscope: RP = 1.22 λ / (2 n sinθ); Telescope: RP = 1.22 λ / D; Smaller λ & larger D → better RP.
Case-based: Given YDSE setup with shift due to sheet or diffraction pattern, find fringe width, shift, or angular separation.	Case-Based (4 marks)	2023, 2025	Repeated 2x; Use β = λ D/d, shift = (μ-1)t D/d.
Why does light from two independent sources not produce sustained interference?	Short Answer (2 marks)	2021 Term 2, 2024	Repeated 2x; No constant phase relation → random phase difference → no fixed pattern.

Best Format for Preparation and Answering

Study Format: Memorize YDSE fringe width, single slit minima, Brewster’s law & resolving power formulas. Practice ray diagrams for interference & diffraction daily. Make table for differences (interference vs diffraction).
Answer Format: Derivations: Use path difference → condition for bright/dark fringes/minima. Numericals: Calculate β, shift, angular width → show steps. Ray diagrams: Draw wavefronts/slit sources → show superposition.
Tips: Young’s double slit (fringe width & shift) & single slit diffraction (central max width) are highest repeat & marks. Always mention coherent sources requirement. Time: 10–12 mins for 4–6 mark questions. Very scoring with neat diagrams & correct formulas.

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