CBSE Class 12 Physics Dual Nature of Radiation and Matter 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 on photoelectric effect/de Broglie wavelength (3-5 marks).
Repeated Topics: Photoelectric effect (Einstein’s equation hf = φ + K_max), threshold frequency/wavelength, stopping potential, intensity effect, graphs (I vs V, K_max vs ν), de Broglie hypothesis & wavelength λ = h/p, Davisson-Germer experiment, wave nature of electrons, particle nature of light (photon energy E = hf).
Difficulty: Medium; photoelectric numericals (stopping potential, threshold) & de Broglie wavelength calculations are most repeated & scoring.
Changes: More numericals on threshold wavelength & kinetic energy with different metals in 2023–2025.

Most Important and Repeated Questions

Question Example	Type/Marks	Years Repeated	Notes
Explain photoelectric effect. Derive Einstein’s photoelectric equation hf = φ + K_max. Explain significance of threshold frequency & work function.	Derivation/Theory (4 marks)	2021, 2022, 2023, 2024, 2025	Repeated 5x; K_max = hf - φ; hf₀ = φ; No emission below threshold; Intensity affects number of electrons, not energy.
Light of wavelength λ is incident on metal surface with work function φ. Find maximum kinetic energy of photoelectrons & stopping potential.	Numerical (3 marks)	2021 Term 2, 2022, 2023, 2024	Repeated 4x; K_max = hc/λ - φ; V₀ = K_max / e (in volts).
Derive de Broglie wavelength λ = h/p for matter waves. Calculate wavelength of electron accelerated by V volts.	Derivation/Numerical (3 marks)	2022, 2023, 2024, 2025	Repeated 4x; λ = h / √(2m e V) ≈ 12.27 / √V Å (for electrons).
Assertion: Photoelectric effect demonstrates particle nature of light. Reason: Energy is absorbed in discrete packets (photons).	Assertion-Reason (1 mark)	2023, 2024, 2025	Repeated 3x; Both true, reason explains.
Graph of stopping potential vs frequency for two metals. Identify which has higher work function & threshold frequency.	Short Answer (2-3 marks)	2021 Term 1, 2022, 2023, 2025	Repeated 4x; Slope = h/e same; Intercept on ν-axis = φ/h → higher intercept → higher φ & ν₀.
Calculate de Broglie wavelength of proton/neutron/alpha particle accelerated by same potential V.	Numerical (2-3 marks)	2022, 2023, 2024	Repeated 3x; λ ∝ 1/√m → electron > proton > alpha (for same V).
MCQ: The de Broglie wavelength is inversely proportional to: (a) velocity (b) momentum (c) energy (d) mass	MCQ (1 mark)	2021 Term 1, 2023, 2024	Repeated 3x; Answer (b) momentum (λ = h/p).
Explain Davisson-Germer experiment to prove wave nature of electrons. Mention observed wavelength matches de Broglie prediction.	Short Answer (3 marks)	2023, 2025	Repeated 2x; Electron diffraction pattern like X-ray → confirms λ = h/p.
Case-based: Given incident wavelength & work function, find stopping potential, max KE, threshold wavelength, or number of photons.	Case-Based (4 marks)	2023, 2025	Repeated 2x; Use E = hc/λ, K_max = E - φ, V₀ = K_max/e.
Why no photoelectric emission below threshold frequency even with high intensity?	Short Answer (2 marks)	2021 Term 2, 2024	Repeated 2x; Photon energy hf < φ → no electron ejection (quantum nature).

Best Format for Preparation and Answering

Study Format: Memorize Einstein’s equation, de Broglie wavelength formula & graphs (K_max vs ν, I vs V). Practice numericals on stopping potential & wavelength daily. Make table for photoelectric effect observations vs wave theory failure.
Answer Format: Derivations: hf = φ + (1/2)mv²_max → K_max = hf - φ. Numericals: Convert wavelength to energy → subtract φ → V₀ = K_max/e. Always mention units (eV for energy, Å for wavelength).
Tips: Photoelectric equation numericals (threshold, stopping potential) & de Broglie wavelength calculations are highest repeat & marks. Learn why wave theory fails (no threshold, instantaneous emission, intensity dependence). Time: 8–10 mins for 3–5 mark questions. Very scoring if equations & graphs are clear.

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