Chapter 10 — Wave Optics

Chapter 10 of the Class 12 Physics NCERT textbook, "Wave Optics", covers Huygens' principle, the wave nature of light, and the phenomena of interference, diffraction, and polarisation, explaining how light behaves as a transverse electromagnetic wave.

• From particles to waves — The chapter traces the shift from Newton's corpuscular model to Huygens' wave theory and Maxwell's electromagnetic view, establishing why light is best understood as a wave.
• Huygens' principle at work — Treating each point on a wavefront as a source of secondary wavelets, the chapter re-derives reflection and refraction, showing the wave model explains what the particle model could not.
• Interference and diffraction — Young's double-slit experiment reveals light interfering to form fringes, while single-slit diffraction shows light spreading and its intensity varying — hallmarks of wave behaviour.
• Polarisation and transverse nature — Because light is a transverse wave, polaroids can filter its vibration direction. Malus' law describes how intensity changes as polarised light passes through crossed polaroids.

1. 01Huygens' principle: each point on a wavefront acts as a source of secondary wavelets; the forward envelope of these wavelets gives the new wavefront position at a later time.
2. 02Snell's law (n₁ sin i = n₂ sin r) is derived from Huygens' principle; when light bends toward the normal on refraction, its speed in the second medium is lower — confirming the wave model over Newton's corpuscular model.
3. 03Young's double-slit experiment (1801) established the wave nature of light; bright fringes form where path difference equals nλ (constructive interference) and dark fringes where path difference equals (n + ½)λ (destructive interference).
4. 04Single-slit diffraction produces a broad central maximum with intensity minima at angles θ where sin θ = nλ/a (n = ±1, ±2, …), and successively weaker secondary maxima between them.
5. 05Light is a transverse electromagnetic wave; a polaroid transmits only the electric-field component along its pass-axis, reducing unpolarised light intensity by half. When polarised light passes through a second polaroid at angle θ, intensity follows Malus' law: I = I₀ cos²θ.
6. 06Total internal reflection occurs when light travels from a denser to a rarer medium and the angle of incidence exceeds the critical angle ic, where sin ic = n₂/n₁.