Practice question

**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. Fringe width β = (λ D/d) . λ = 5.3 × 10⁻⁷ m , d = 1.5 × 10⁻⁴ m , D = 1.8 m . β = (5.3 × 10⁻⁷ × 1.8/1.5 × 10⁻⁴) = 6.36 × 10⁻³ m = 6.36 mm . Using Δ = d sinθ, y = n λ D/d,