Why does the wave theory predict that light bends towards the normal when entering a medium where its speed is lower?
**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. The wavefront tilts towards the normal because the part entering the slower medium lags, reducing the angle of propagation relative to the boundary. 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 Wavefront
Ref: NCERT > Physics Book > Wave Optics > Wavefront and Huygens Principle