Section 7 — Wave Optics: Huygens' Principle
GE-4 / PHYS7021 unit 7 · 2 lecture-hours · 1 solved question (+1 combined)
7.1 Theory you need
- Light is an electromagnetic wave (transverse) — its optical behaviour (reflection, refraction, interference…) follows from the wave nature.
- Wavefront: locus of points vibrating in the same phase (plane from distant source, spherical from point source); rays ⊥ wavefront.
7.2 Solved past questions — exam-style answers
1. State (and explain) Huygens' principle. (2024: "What is dual character of light?" + state Huygens' principle)
- Principle (statement): (i) every point of a wavefront acts as a source of secondary spherical wavelets travelling with the speed of the wave; (ii) the forward envelope (common tangent surface) of these wavelets at a later instant gives the new position of the wavefront.
- Explanation: the construction correctly predicts rectilinear propagation, reflection and refraction; with Fresnel's addition of interference it also explains diffraction.
- (2024 extra half) Dual character of light: light shows wave behaviour (interference, diffraction, polarization) and particle behaviour (photoelectric effect, Compton effect) — photons of energy hν. Note: the dual-character half is outside PHYS7021; Huygens' principle is in your syllabus.
∴ Every wavefront point → secondary wavelets; envelope = new wavefront