CBSE Class 12 Physics: Wave Optics — Interference & Diffraction 2026

Huygens' Principle

• Every point on a wavefront is a source of secondary wavelets; envelope of wavelets = new wavefront
• Explains refraction: wavefront bends because speed changes at interface
• Explains reflection: angle of incidence = angle of reflection from wavefront geometry

Coherent Sources and Interference

• Coherent sources: same frequency, constant phase difference
• Constructive interference: path difference = nλ (bright fringe); destructive: (n+½)λ (dark fringe)
• Young's double slit: fringe width β = λD/d; D = screen distance, d = slit separation

Young's Double Slit Experiment

• Conditions for sustained interference: coherent, monochromatic, narrow sources
• Fringe width β = λD/d; increases with λ and D; decreases with d
• Intensity distribution: I = 4I₀cos²(φ/2) where φ = phase difference

Diffraction

• Bending of waves around obstacles/edges; significant when obstacle size ≈ λ
• Single slit: central maximum width 2λD/a (a = slit width); minima at a sin θ = nλ
• Resolving power of optical instruments limited by diffraction (Rayleigh criterion)

Polarisation

• Transverse waves can be polarised; longitudinal waves (sound) cannot be polarised
• Plane polarisation by: transmission (Polaroid), reflection (Brewster's law: tan θ_B = n), scattering
• Malus's law: I = I₀cos²θ; θ = angle between polariser and analyser

Wave vs. Particle Nature of Light

• Photoelectric effect supports particle nature; interference/diffraction support wave nature
• Wave optics successfully explains phenomena particle optics cannot
• Quantum mechanics unifies both: wave-particle duality

CBSE Exam — Wave Optics

• Derive expression for fringe width in Young's double slit experiment
• Sodium light (λ=589nm): D=1.5m, d=0.5mm; find fringe width and position of 3rd bright fringe
• State and verify Malus's law; explain how polaroids are used in sunglasses