Light and Waves
Core concepts
Waves transfer energy without transferring matter. Key quantities are wavelength, frequency, period, amplitude and speed, linked by v = fλ. Mechanical waves need a medium; electromagnetic waves do not and travel at c in a vacuum. Transverse and longitudinal waves differ in oscillation direction relative to propagation.
Superposition explains interference: constructive interference occurs where waves arrive in phase (path difference nλ), destructive where out of phase ((n + ½)λ). Young's double-slit experiment gives fringe spacing Δy = λL/d. Diffraction is greatest when wavelength is comparable to the gap size.
Standing waves form when waves of equal frequency travelling in opposite directions superpose, producing nodes and antinodes. For strings fixed at both ends, λ = 2L/n; for pipes, open and closed ends give different harmonic series.
The wave and particle models of light are both needed: polarisation, diffraction and interference support the wave model, while the photoelectric effect supports photons of energy E = hf. Refraction follows Snell's law, n₁sinθ₁ = n₂sinθ₂, with total internal reflection occurring beyond the critical angle.
Traps
Mixing up frequency and wavelength when a wave changes medium (frequency stays constant); forgetting path difference conditions; using the wrong harmonic formula for open versus closed pipes; confusing intensity with frequency in photoelectric questions; assuming light always behaves as a wave.
Worked example
Light of wavelength 600 nm passes through double slits 0.30 mm apart onto a screen 2.0 m away. Fringe spacing Δy = λL/d = (600 × 10⁻⁹ × 2.0)/(0.30 × 10⁻³) = 4.0 × 10⁻³ m, or 4.0 mm. Rule of thumb: convert all lengths to metres before substituting.