Class 12 Physics Chapter 10 Wave Optics – Revision Notes

Quick revision notes for Class 12 Physics Chapter 10 – Wave Optics, covering Huygens’ principle, wavefronts, and Young’s double-slit interference — ideal for last-minute board exam revision.

Huygens’ Principle

Every point on a wavefront becomes a source of secondary spherical wavelets; the envelope of these wavelets at a later instant gives the new wavefront. Used to derive the laws of reflection and refraction from the wave theory of light, and to explain why a wavefront bends towards the normal when entering a denser (slower) medium.

Wavefronts

Spherical wavefront: from a point source, radius increasing with distance. Plane wavefront: from a source at infinity (or a point source at the focus of a converging lens/mirror) — parallel rays, flat wavefronts perpendicular to them. Cylindrical wavefront: from a line source.

Refraction and Reflection (Wave Picture)

Snell’s law n₁ sin i = n₂ sin r follows from Huygens’ construction, with speed v₁=c/n₁ in the first medium and v₂=c/n₂ in the second. Frequency is fixed by the source and never changes on reflection or refraction; wavelength and speed both change with the medium (λ=v/ν).

Interference — Young’s Double-Slit Experiment

Path difference at a point on the screen, distance y from the centre, slit separation d, screen distance D (with D>>d): path difference ≈ yd/D.

Constructive interference (bright fringe): path difference = nλ (n=0,±1,±2,…) ⇒ y_n=nλD/d.

Destructive interference (dark fringe): path difference = (n+½)λ.

Fringe width: β=λD/d (spacing between consecutive bright, or consecutive dark, fringes — independent of n).

Intensity at any point: I=4I₀cos²(φ/2), where φ=(2π/λ)×(path difference) is the phase difference, and I₀ is the intensity from each slit alone. Maximum intensity 4I₀ occurs at bright fringes; minimum (zero) at dark fringes.

Coherent sources (constant phase relationship) are essential for a stable, observable interference pattern — two independent ordinary sources cannot produce visible fringes since their relative phase fluctuates randomly.

One-Line Summary

Chapter 10 shifts from the ray picture of light to the wave picture — Huygens’ principle explains reflection and refraction as wavefront phenomena, and interference in Young’s double-slit experiment demonstrates the wave nature of light directly, with fringe position and width governed by path difference, wavelength, slit separation, and screen distance.