Why does the interference pattern from two slits require light to originate from a single source?
**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. A single source ensures coherence, maintaining a constant phase relationship between the waves from the slits, essential for a stable interference pattern. 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 To
Ref: NCERT > Physics Book > Wave Optics > Wavefront and Huygens Principle