Wave Optics — Class 12 Physics

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 , bright where the path difference is an integral multiple of and dark where it is an odd multiple of . 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 ; 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 , and on reflection at Brewster's angle . Together these effects confirm light's wave character and underpin modern optical technology.