Wave Optics

Every point on a wavefront acts as a source of secondary wavelets. The new wavefront is the tangent to all secondary wavelets. Explains: reflection, refraction, interference, diffraction — things that particle theory of light CANNOT explain. Speed of light in medium = c/n where n = refractive index. λ_medium = λ_vacuum/n.

When two coherent light waves meet, they superpose. Constructive interference: path difference = nλ → bright fringe. Destructive interference: path difference = (2n+1)λ/2 → dark fringe. Coherent sources: same frequency, constant phase difference. White light: colored fringes because different λ diffract differently.

Two slits S₁ and S₂ separated by d, screen at distance D. Fringe width β = λD/d. Position of nth bright fringe: y_n = nλD/d. Position of nth dark fringe: y_n = (2n-1)λD/2d. Angular fringe width = λ/d. Fringe pattern: alternating bright and dark bands of equal width.

Diffraction: bending of light around obstacles/slits. Single slit: central bright maximum (widest), secondary maxima. Condition for dark fringe: a sinθ = nλ (a=slit width). Polarization: transverse nature of light. Polaroid transmits only one plane of vibration. Brewster's law: tanθ_B = n (θ_B = polarizing angle).