Practice question

**Convex lens focusing** plane wave into point because lens introduces phase delay proportional to thickness, converting plane wavefront to spherical converging to focal point, property ensures rays parallel to axis meet at focus, spherical aberration minimized for paraxial rays, lensmaker's formula determines focal length. sin i_c = (n₂/n₁) , where n₂ = 1.0 (air), i_c = 39° . sin 39° ≈ 0.629 , n₁ = (1.0/0.629) ≈ 1.59 . 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 1.59,