Why does the interference pattern from two slits vanish if one slit is covered?
**Wavefront types** point source spherical, distant point source plane, after convex lens plane wave focuses to point because lens adds phase delay proportional to thickness, converging spherical wavefront, after concave mirror plane wave becomes spherical converging to focus, illustrating Huygens construction. Interference requires superposition from two sources; covering one slit eliminates the second wave, leaving only a diffraction pattern from the single slit. Using Δ = d sinθ, y = n λ D/d, a sinθ = n λ, I = I₀ cos²θ, n = c/v, sinC = 1/n, λ' = λ/n and A = 2a cos(φ/2), calculation gives On
Ref: NCERT > Physics Book > Wave Optics > Wavefront and Huygens Principle