CBSE Class 12 — Notes, Chapters & Practice Quizzes

Chapter 2: Wave Optics

Summary

Wave optics explains phenomena that ray optics cannot, by treating light as a wave. Huygens' principle, in which every point on a wavefront is a source of secondary wavelets, derives the laws of reflection and refraction. When two coherent waves overlap they interfere; Young's double-slit experiment produces alternating bright and dark fringes of spacing \(\beta=\dfrac{\lambda D}{d}\), bright where the path difference is an integral multiple of \(\lambda\) and dark where it is an odd multiple of \(\lambda/2\). Coherence—a constant phase relationship—is essential for stable fringes. Diffraction, the bending of light around obstacles and slits, gives a central bright maximum flanked by weaker minima, with the first minimum at \(a\sin\theta=\lambda\); this sets the ultimate resolution limit of optical instruments. Polarisation reveals the transverse nature of light: unpolarised light becomes plane-polarised on passing through a Polaroid, obeying Malus' law \(I=I_0\cos^2\theta\), and on reflection at Brewster's angle \(\tan i_B=n\). Together these effects confirm light's wave character and underpin modern optical technology.

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