Superposition · revision map · A-Level 9702, Topic 8

The principle of superposition: where waves meet, their displacements add (y = y₁ + y₂).

• Two equal waves travelling in opposite directions (incident plus reflected) form a stationary wave.
• Fixed nodes (N) never move; antinodes (A) swing with greatest amplitude. No net energy travels along it.
• Adjacent nodes are half a wavelength apart.

Waves spread out when they pass through a gap or around an edge.

• Every wave bends around obstacles and edges.
• Spreading is greatest when the gap width is about equal to the wavelength λ.
• Wavelength and frequency are unchanged.
• Diffraction at a single slit precedes the patterns from two slits and a grating.

Coherent sources (constant phase difference, same λ) build a fixed fringe pattern.

• Path difference = mλ → constructive (bright).
• Path difference = (m + ½)λ → destructive (dark).
• Young's double slit fixes λ from the fringe spacing x.

Many slits sharpen the maxima into well-separated orders, so λ is measured precisely.

• Slit spacing d comes from the lines per metre; n is the order.
• Maxima are bright and far apart, giving a precise λ.
• White light splits into spectra: each λ at its own angle θ.