What is the direction of rays relative to the wavefront in a light wave?
**Huygens principle** predicts shape of wavefront after propagation, for point source close spherical, far plane, after reflection from plane mirror spherical wave becomes spherical with centre mirrored, plane wave remains plane but direction changes angle of incidence equals reflection, after passing through thin prism plane wavefront tilts due to different path. Rays are perpendicular to the wavefront, representing the direction of energy propagation. 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 Perpendicular,
Ref: NCERT > Physics Book > Wave Optics > Wavefront and Huygens Principle