What happens to the fringe width in a double-slit experiment if both the slit separation and screen distance are doubled
**Coherence** requires constant phase difference, two independent sources not coherent because phase random, prevents interference pattern, laser coherent, visibility of fringes determined by coherence, intensity and phase difference, coherent sources produce stable interference, incoherent intensity adds I = I₁+I₂ no interference, interference pattern distinguishes from diffraction. Fringe width β = (λ D/d) . If D and d are both doubled, β = (λ (2D)/2d) = (λ D/d) , so it remains the same. 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
Ref: NCERT > Physics Book > Wave Optics > Interference - Double-Slit and Coherence