• Tags: YDSE, setup, fringe-width
• Difficulty: Foundation

In YDSE, monochromatic light illuminates two narrow slits $S_{1}$ and $S_{2}$ separated by distance d. The screen is at distance D (D >> d). Path difference at point P on the screen: $\Delta x$ = $S_{2}$P - $S_{1}$P = d sinθ ≈ dy/D (for small θ). Bright fringes form where $\Delta x$ = nλ (constructive interference): y_n = nλD/d. Dark fringes form where $\Delta x$ = (2n-1)λ/2 (destructive): y_n = (2n-1)λD/(2d). The fringe width β = λD/d is uniform across the screen. The central fringe (n=0) is always bright and equidistant from both slits. The angular fringe width θ = λ/d is independent of D.