• summary_type: concept
• word_count: 200

Young's Double Slit Experiment demonstrates interference of coherent light. Two slits (separation d) illuminated by a monochromatic source produce an interference pattern on a screen (distance D). Path difference at point y from center: $\Delta x$ = dy/D. Bright fringes at $\Delta x$ = nλ (positions y = nλD/d). Dark fringes at $\Delta x$ = (2n-1)λ/2. Fringe width β = λD/d is uniform across the screen. Intensity distribution: I = $4I_{0}$c$os^{2}$(φ/2) where φ = 2πdy/(λD). For unequal intensities: I_max = (√$I_{1}$ + √$I_{2}$)^{2}, I_min = (√$I_{1}$ - √$I_{2}$)^{2}. The ratio I_max/I_min = ((a_{1}+a_{2})/(a_{1}-a_{2}))^{2} where a_{1}, a_{2} are amplitudes. Key modifications: (1) glass slab over one slit shifts central fringe by (μ-1)tD/d toward the slab without changing fringe width; (2) in a medium of index n, fringe width becomes β/n; (3) with white light, the central fringe is white and higher orders are colored. These results form the foundation of nearly all JEE wave optics problems.