Wave Optics Class 12 Notes | CBSE Physics Chapter 10

Wave Optics is Chapter 10 of CBSE Class 12 Physics. This chapter treats light as a wave and explains phenomena that cannot be explained by ray optics — interference, diffraction, and polarisation. Huygens’ principle, Young’s double slit experiment, and single slit diffraction are the core topics.

This chapter carries 5–7 marks. Young’s double slit experiment derivation, diffraction, and polarisation are the most tested topics.

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Key Concepts

1. Huygens’ Principle

Every point on a wavefront acts as a source of secondary spherical wavelets. The new wavefront is the forward envelope of all these secondary wavelets.

Used to derive the laws of reflection and refraction.

2. Interference — Young’s Double Slit Experiment (YDSE)

When light from two coherent sources (slits S₁ and S₂) overlaps, it produces a pattern of bright and dark fringes.

Condition for bright fringe: Path difference = nλ (n = 0, 1, 2, …)

Condition for dark fringe: Path difference = (n + ½)λ

Fringe width: β = λD/d

• λ = wavelength of light
• D = distance from slits to screen
• d = separation between slits

Central fringe is always bright and widest.

3. Diffraction — Single Slit

When light passes through a narrow slit, it spreads out and forms a pattern of bright and dark bands.

Central maximum: Width = 2λD/a (where a = slit width)

Minima condition: a sin θ = nλ (n = ±1, ±2, …)

Secondary maxima: a sin θ = (n + ½)λ

Central maximum is twice as wide as secondary maxima.

4. Polarisation

Light is a transverse wave. Polarisation is the restriction of vibrations of the electric field to a single plane.

Malus’s Law: I = I₀ cos² θ (intensity of polarised light through an analyser at angle θ)

Brewster’s Law: tan ip = n (ip = polarising angle; reflected and refracted rays are perpendicular)

Polaroid uses: Sunglasses, LCD screens, photography, 3D movies

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Solved Examples

Example 1

In YDSE, slit separation d = 0.5 mm, screen distance D = 1 m, wavelength λ = 600 nm. Find the fringe width.

Answer: β = λD/d = (600 × 10⁻⁹ × 1)/(0.5 × 10⁻³) = 1.2 mm

Example 2

Unpolarised light of intensity I₀ passes through two polaroids with axes at 60°. Find final intensity.

Answer: After first polaroid: I₁ = I₀/2. After second: I₂ = (I₀/2)cos²60° = (I₀/2)(1/4) = I₀/8

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Important Questions for Board Exams

3-Mark

1. Derive the expression for fringe width in YDSE.
2. State and prove Malus’s law.
3. What is diffraction? Compare the diffraction pattern with interference pattern.

5-Mark

1. Describe Young’s double slit experiment. Derive expressions for bright, dark fringes and fringe width.
2. What is polarisation? State Brewster’s law and Malus’s law with derivations.

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Quick Revision Points

• Huygens: each point on wavefront → source of secondary wavelets
• YDSE: β = λD/d; bright: Δ = nλ; dark: Δ = (n+½)λ
• Diffraction: central max width = 2λD/a; minima: a sin θ = nλ
• Polarisation: transverse wave property; Malus: I = I₀cos²θ
• Brewster: tan ip = n; reflected light is fully polarised

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Previous: Ch 9 — Ray Optics
Next: Ch 11 — Dual Nature