Waves and Wave Phenomena
What the exam tests
This domain covers superposition, interference, diffraction, stationary waves and sound. Questions often require path-difference conditions, Young's double-slit or diffraction-grating calculations, and node/antinode positions in stationary waves.
Core rules
- Wave speed v = fλ; period T = 1/f.
- Superposition adds displacements. Constructive interference occurs when path difference = nλ; destructive when path difference = (n + ½)λ.
- Young's double slits: fringe spacing x = λD/d, where d is slit separation and D is slit-to-screen distance.
- Diffraction grating: d sin θ = nλ; d is the grating spacing.
- A stationary wave has nodes separated by λ/2 and antinodes halfway between nodes.
- Sound requires a medium; intensity is power per unit area and falls with distance from a point source.
Worked example
In a Young's double-slit experiment, λ = 600 nm, slit separation d = 0.50 mm and screen distance D = 1.5 m. Find the fringe spacing. x = λD/d = (600 × 10⁻⁹ × 1.5)/(0.50 × 10⁻³) = 1.8 × 10⁻³ m = 1.8 mm. Adjacent bright fringes are therefore 1.8 mm apart. If D doubles, the spacing doubles; if d doubles, the spacing halves. The small-angle approximation is acceptable because x is much smaller than D.
Traps
Using slit width instead of slit separation; mixing nm and mm; using degrees in a formula requiring radians; counting path difference in metres instead of wavelengths; stating that adjacent nodes are λ apart rather than λ/2; treating a stationary wave as transferring energy along the string.