Exercise Solutions Available
Step-by-step answers for all textbook questions in this chapter
Overview
Wave optics treats light as a wave to explain interference, diffraction and polarisation — phenomena ray optics cannot.
1. Huygens' Principle
Every point on a wavefront acts as a source of secondary wavelets; the new wavefront is their common tangent. It explains reflection and refraction in wave terms.
2. Interference (Young's Double Slit)
3. Diffraction
Bending of light around edges/slits. For a single slit, the first minimum is at a sin θ = λ (a = slit width). Diffraction increases when the slit size is comparable to λ.
4. Polarisation
Worked Examples
Example 1 — Fringe width
λ = 600 nm, d = 0.3 mm, D = 1 m. Find β.
Example 2 — Fringe position
Position of the 3rd bright fringe in the above.
Example 3 — Malus's law
Polarised light of intensity I₀ passes through an analyser at 60°. Find output intensity.
Example 4 — Brewster's angle
Polarising angle for glass of n = 1.5.
Common Mistakes
- Using λ/2 for bright fringes (it is nλ).
- Not converting nm and mm to metres.
- Forgetting the square in Malus's law (cos²θ).
- Confusing interference (two coherent sources) with diffraction (single slit/edge).
Exam Tips & Revision Checklist
- β = λD/d — fringes are equally spaced.
- Coherent sources (constant phase difference) are essential for steady interference.
- Practised: fringe width, fringe position, Malus's law, Brewster's angle.