Wave Optics
What this domain tests
Wave Optics tests Huygens' principle, interference, Young's double slit, diffraction at a single slit, polarisation and resolving power.
Core rules
- Superposition: constructive path difference nλ; destructive path difference (n + ½)λ.
- Young's fringes: fringe width β = λD/d; intensity varies as cos²(φ/2).
- Diffraction minima: a sinθ = nλ; the central maximum is twice as wide as the other maxima.
- Polarisation: Malus's law I = I0 cos²θ; Brewster angle tanθ_B = n.
- Steady interference requires coherent sources with a constant phase difference.
Worked example
In a double-slit experiment λ = 600 nm, slit separation d = 0.50 mm and screen distance D = 1.0 m. β = λD/d = (600×10⁻⁹ × 1.0)/(0.50×10⁻³) = 1.2×10⁻³ m = 1.2 mm. If d is doubled, fringe width halves; if the apparatus is immersed in water, β decreases because the wavelength decreases.
Traps
- Mixing interference conditions for two slits with diffraction conditions for one slit.
- Using the wrong path difference for dark fringes; it is an odd multiple of λ/2.
- Forgetting that fringe width depends on wavelength, so colour changes the pattern.
- Believing polarisation proves longitudinal waves; it demonstrates the transverse nature of light.