• summary_type: concept
• word_count: 170

The intensity at any point in YDSE depends on the phase difference φ = 2π$\Delta x$/λ. For equal intensity sources: I = $4I_{0}$c$os^{2}$(φ/2). Important benchmarks: at φ = 0 (center), I = $4I_{0}$; at φ = π/2 (path diff λ/4), I = $2I_{0}$; at φ = 2π/3 (path diff λ/3), I = $I_{0}$; at φ = π (path diff λ/2), I = 0. For unequal slit widths w_{1} and w_{2}, intensity is proportional to slit width (I ∝ w), so $I_{1}$/$I_{2}$ = w_{1}/w_{2}. The fringe visibility V = (I_max - I_min)/(I_max + I_min) = 2√(I_{1}$$I_{2})/($I_{1}$ + $I_{2}$) ranges from 0 (no fringes) to 1 (perfect dark minima). Maximum visibility occurs when $I_{1}$ = $I_{2}$. Energy is conserved — the average intensity over the full pattern equals $I_{1}$ + $I_{2}$, with energy merely redistributed from minima to maxima.