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Essential formulas: YDSE: β = λD/d, y_n = nλD/d (bright), y_n = (2n-1)λD/2d (dark), I = $4I_{0}$c$os^{2}$(φ/2), φ = 2πdy/(λD), shift = (μ-1)tD/d. Unequal slits: I_max/I_min = ((a_{1}+a_{2})/(a_{1}-a_{2}))^{2}. Single slit: minima at a sinθ = nλ, central width = 2λD/a. Circular aperture: θ_min = 1.22λ/D. Malus: I =$I_{0}$c$os^{2}$θ. Brewster: tan i_B = n. Grating: d sinθ = nλ.

JEE strategy: (1) In YDSE, always identify modifications (slab, medium, source shift) before applying formulas. (2) Never confuse interference and diffraction conditions — nλ gives bright fringes in interference but dark bands in diffraction. (3) For intensity problems, convert path difference to phase difference first, then use c$os^{2}$. (4) For polarization, apply Malus's law sequentially through each polaroid. (5) Remember that the first polaroid always halves unpolarized light intensity. (6) In thin film problems, count the number of π phase changes to determine whether to add λ/2 to the path difference. Wave optics carries about 2.3% JEE weightage, typically 1 MCQ.$