CBSE Class 12 Physics Ray Optics and Optical Instruments 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), 2-3 numericals/derivations on lenses/mirrors/prism/instruments (3-6 marks).
Repeated Topics: Reflection/refraction at spherical surfaces (mirror & lens formula, sign convention), prism deviation & dispersion (minimum deviation angle), total internal reflection & applications (optical fibre, mirage), optical instruments (microscope, astronomical telescope, resolving power), ray diagrams, power of lens combination, scattering (blue sky, red sunset), rainbow formation.
Difficulty: Medium to high; numericals on lens/mirror combination & microscope/telescope magnification are most repeated & scoring.
Changes: More numericals on power of combination of lenses & prism deviation in 2023–2025.

Most Important and Repeated Questions

Question Example	Type/Marks	Years Repeated	Notes
Derive lens maker’s formula for thin lens. Also derive mirror formula using sign convention.	Derivation (4 marks)	2021, 2022, 2023, 2024, 2025	Repeated 5x; Lens maker: 1/f = (μ-1)(1/R1 - 1/R2); Mirror: 1/f = 1/v + 1/u (Cartesian sign convention).
A convex lens of focal length f forms image of object at distance u. Find magnification, nature of image. Also find power when two lenses combined.	Numerical (3 marks)	2021 Term 2, 2022, 2023, 2024	Repeated 4x; m = v/u = f/(f-u); P = 1/f (in dioptre); Combination P = P1 + P2 - d P1 P2 (d in m).
Explain total internal reflection. Derive critical angle formula. Give applications: optical fibre, mirage, diamond brilliance.	Short Answer (3 marks)	2022, 2023, 2024, 2025	Repeated 4x; sin C = 1/μ; Conditions: denser to rarer medium, i > C.
Assertion: Sky appears blue due to scattering. Reason: Scattering ∝ 1/λ⁴ (Rayleigh scattering).	Assertion-Reason (1 mark)	2023, 2024, 2025	Repeated 3x; Both true, reason explains (blue light scatters more).
Draw ray diagram for image formation by compound microscope. Derive magnification m = m₀ × m_e = - (L/f₀)(1 + D/fe).	Ray Diagram + Derivation (4 marks)	2021 Term 1, 2022, 2023, 2025	Repeated 4x; Objective real inverted image → eyepiece virtual magnified; L = tube length.
Find minimum deviation angle for prism (μ, A given). Also find deviation for small angle prism.	Numerical/Derivation (3 marks)	2022, 2023, 2024	Repeated 3x; δ_min = 2 sin⁻¹(μ sin(A/2)) - A; For small A: δ = (μ-1)A.
MCQ: Resolving power of telescope depends on: (a) focal length of objective (b) aperture diameter (c) eyepiece power (d) tube length	MCQ (1 mark)	2021 Term 1, 2023, 2024	Repeated 3x; Answer (b) aperture diameter (D); RP ∝ D/λ.
Explain formation of rainbow. Why primary rainbow has red outside & violet inside?	Short Answer (3 marks)	2023, 2025	Repeated 2x; Refraction + reflection + dispersion in raindrops; Red deviates less → outside in primary.
Case-based: Given lens/mirror combination or optical instrument, find focal length, magnification, nature of image.	Case-Based (4 marks)	2023, 2025	Repeated 2x; Use lens/mirror formula step-by-step.
Why stars twinkle & planets do not? Explain atmospheric refraction & scattering.	Short Answer (2 marks)	2021 Term 2, 2024	Repeated 2x; Refraction in varying density layers → position shift → twinkling; Planets closer & larger disk → less effect.

Best Format for Preparation and Answering

Study Format: Memorize lens/mirror formula, prism deviation, microscope/telescope magnification formulas. Practice ray diagrams & numericals on combination daily. Make table for optical instruments & spectrum applications.
Answer Format: Derivations: Use sign convention → paraxial ray approximation → formula. Ray diagrams: Draw principal rays → locate image. Numericals: Show u, v, f values → calculate m, nature. Always mention real/virtual, erect/inverted.
Tips: Lens/mirror formula applications & optical instruments (magnification & ray diagram) are highest repeat & marks. Practice sign convention carefully (Cartesian). Time: 10–12 mins for 4–6 mark questions. Very scoring if diagrams neat & calculations accurate.

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