CBSE · Class 12 · Physics · Chapter 10

Wave
Optics

Complete chapter resources for CBSE Class 12 Physics — topic breakdown, key formulas, sample questions, previous year board questions, and instant AI question paper generation.

4Topics
6–8Board marks
8Sample questions
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Key Formulas — Chapter 10
  • Fringe width (YDSE): β = λD / d
  • Path diff. bright fringe: Δ = nλ (n = 0, 1, 2, …)
  • Path diff. dark fringe: Δ = (2n−1)λ/2
  • Single-slit min. (diffraction): a sin θ = mλ
  • Brewster's law: tan i_B = n
  • Malus's law: I = I₀ cos² θ
Chapter Overview

What this chapter covers

Wave Optics treats light as a wave rather than a ray, explaining phenomena that geometrical optics cannot. The chapter begins with Huygens principle — every point on a wavefront is a source of secondary wavelets — and uses it to derive the laws of reflection and refraction rigorously, showing that wave theory is consistent with the familiar ray-optic results.

The core of the chapter is Young's Double-Slit Experiment (YDSE), which demonstrates that coherent light sources produce stable interference fringes. Students learn to calculate fringe width (β = λD/d), positions of bright and dark bands, and how changing wavelength, slit separation, or screen distance shifts the pattern. Single-slit diffraction extends these ideas: a single slit of width a produces a wide central maximum flanked by weaker secondary maxima, and minima occur when a sin θ = mλ.

The chapter closes with polarisation of light — a property unique to transverse waves. Brewster's law (tan i_B = n) gives the polarising angle at which reflected light is completely plane-polarised, and Malus's law (I = I₀ cos² θ) governs the intensity of polarised light passed through an analyser. Board questions on polarisation are typically 2–3 marks and appear frequently across all paper sets.

Topics

What's inside Chapter 10

As per NCERT Class 12 Physics Part II (CBSE syllabus)

Topic 1
Huygens Principle & Wave Theory of Light
Wavefront concept, secondary wavelets, and the envelope principle. Derivation of laws of reflection and refraction using Huygens construction. Comparison with corpuscular theory.
Topic 2
Coherence & Interference of Light
Conditions for sustained interference — coherent sources, constant phase difference. Young's double-slit experiment: fringe width β = λD/d, path difference analysis, effect of λ, d, and D on fringe pattern.
Topic 3
Diffraction of Light
Single-slit diffraction pattern: central maximum width = 2λD/a, minima at a sin θ = mλ. Comparison with double-slit interference. Resolving power of optical instruments (conceptual).
Topic 4
Polarisation of Light
Transverse nature of light. Plane polarised light by reflection (Brewster's law: tan i_B = n), by scattering, and by polaroids. Malus's law: I = I₀ cos² θ. Applications of polaroids.
Connections

How this chapter fits in

Useful for setting question difficulty and cross-chapter papers.

Builds on
Ch 9 · Ray Optics
Reflection, refraction, lenses — extended to wave treatment
Ch 8 · Electromagnetic Waves
Transverse nature of light, speed c = 1/√(μ₀ε₀)
Chapter 10 Wave
Optics
Leads to
Ch 11 · Dual Nature of Radiation
Photoelectric effect — wave vs. particle duality of light
Class 12 Chemistry — Spectroscopy
Emission/absorption spectra understood via wave interference
Board Blueprint

Marks & question-type breakdown

Typical pattern based on CBSE Class 12 Physics board papers from the last five years.

Question type Marks Typical count What's usually tested
MCQ / Assertion–Reason 1 1–2 Nature of wavefront, conditions for interference, or Malus's law
Very Short Answer 2 1 State Huygens principle, Brewster's law, or define coherent sources
Short Answer / Numerical 3 1 Fringe width calculation (YDSE), effect of immersing apparatus in water
Long Answer / Derivation 5 1 Derive fringe width from YDSE geometry, or diffraction pattern analysis
Total (approximate) 6–8 4–5 Weightage varies across paper sets and years
Sample Questions

8 sample questions — generated by MarksZen AI

Aligned to CBSE Class 12 Physics Chapter 10. Covers all question types across Easy, Medium, and Hard difficulty.

Q1 Easy 1 mark MCQ
In Young's double-slit experiment, if the distance between the slits is halved and the distance between the slits and the screen is doubled, the fringe width will: (a) remain unchanged (b) become half (c) become double (d) become four times
Q2 Easy 2 marks Short Answer
State Huygens principle. How does it explain the propagation of a plane wavefront?
Q3 Medium 2 marks Short Answer
In a YDSE, the two slits are separated by 0.3 mm and the screen is placed 1.5 m away. The wavelength of light used is 600 nm. Calculate the fringe width.
Q4 Medium 3 marks Short Answer
State and prove Brewster's law. What happens to the reflected and refracted rays at the polarising angle?
Q5 Medium 3 marks Numerical
Light of wavelength 500 nm falls on a single slit of width 0.1 mm. The diffraction pattern is observed on a screen 2 m away. Find: (i) the angular width of the central maximum (ii) the width of the central maximum on the screen.
Q6 Hard 4 marks Derivation
Using Huygens wave theory, derive the laws of refraction (Snell's law). How does the wave theory explain the decrease in speed of light when it travels from a rarer to a denser medium?
Q7 Hard 5 marks Long Answer
With a neat diagram, derive an expression for the fringe width in Young's double-slit experiment. How does the fringe width change when: (i) the wavelength of the source is increased? (ii) the entire apparatus is immersed in water of refractive index 4/3?
Q8 Hard 5 marks Case-Based
A beam of plane-polarised light of intensity I₀ passes through two polaroids P₁ and P₂. P₂ is rotated with respect to P₁. (i) Write the expression for the intensity of light transmitted by P₂ (Malus's law). (ii) At what angle between P₁ and P₂ will the transmitted intensity be (a) maximum and (b) zero? (iii) If the intensity after P₂ is I₀/4, find the angle between the two polaroids.
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Previous Year Questions

From CBSE board examinations

Actual questions from past Class 12 Physics board papers — Wave Optics chapter.

Board 20233 marks
In a Young's double-slit experiment, the slits are separated by 0.5 mm and the screen is 1.0 m away. The wavelength of light used is 600 nm. Find the fringe width. If the source of light is replaced by one of wavelength 400 nm, what will be the new fringe width? (All India 2023)
Board 20222 marks
State Brewster's law. The polarising angle for a transparent medium is 60°. Find the refractive index of the medium and the angle of refraction at that angle. (Delhi 2022)
Board 20205 marks
Draw a labelled diagram of Young's double-slit experiment. Derive the expression for fringe width. A beam of light consisting of two wavelengths 650 nm and 520 nm is used to illuminate the double slit. Find the least distance from the central maximum at which a bright fringe of one wavelength coincides with a bright fringe of the other. (Slit separation d = 2 mm, D = 120 cm.) (CBSE 2020)
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FAQ

Questions teachers ask

How many marks does Wave Optics carry in the CBSE Class 12 Physics board exam? +
Wave Optics typically carries 6–8 marks in the CBSE Class 12 Physics board exam. This usually appears as one 2-mark short-answer question (often on Huygens principle or conditions for interference) and one 5-mark long-answer or numerical question on Young's double-slit experiment or diffraction. The exact distribution varies across paper sets and years.
What is Huygens principle and why is it important for board exams? +
Huygens principle states that every point on a wavefront acts as a source of secondary wavelets that spread in all directions with the speed of the wave; the new wavefront is the common tangent (envelope) to all secondary wavelets. It is the theoretical foundation of the chapter and is tested in 2-mark questions asking students to state the principle and use it to derive the laws of reflection or refraction.
What is the difference between constructive and destructive interference? +
Constructive interference occurs when two coherent waves meet in phase (path difference = nλ, where n = 0, 1, 2, …), producing bright fringes with maximum intensity. Destructive interference occurs when they meet out of phase (path difference = (2n−1)λ/2), producing dark fringes with zero intensity. In Young's double-slit experiment, fringe width β = λD/d depends on wavelength λ, slit separation d, and screen distance D.
How is single-slit diffraction different from double-slit interference for board purposes? +
In single-slit diffraction the central maximum has width 2λD/a (where a is slit width) and is flanked by progressively weaker secondary maxima. Minima occur at path difference = mλ (m = ±1, ±2, …). In double-slit interference the fringes are equally spaced and of equal width β = λD/d. Board questions often ask students to compare central-maximum widths or calculate fringe/minimum positions, so knowing both sets of conditions is essential.
How do I generate a custom question paper for Wave Optics using MarksZen? +
Sign up for a free MarksZen account, choose CBSE Class 12 Physics, select Chapter 10 (Wave Optics), set your preferred question-type mix and total marks — the AI generates a complete board-aligned paper with answer key in under 2 minutes, ready for PDF export.