Waves, Sound and Light
What the exam tests
Waves, Sound and Light tests wave properties, the wave equation, reflection, refraction, diffraction, interference, sound intensity and harmonics, and the behaviour of light through lenses, prisms and diffraction gratings. Expect calculations plus descriptions of experiments.
Core rules
- v = fλ; frequency is set by the source and does not change on refraction, but speed and wavelength do.
- Angles in optics are always measured from the normal.
- Snell's law: n₁sinθ₁ = n₂sinθ₂; refractive index n = c/v.
- Critical angle: sinC = 1/n, only when going from a denser to a less dense medium.
- Diffraction grating: nλ = d sinθ, where d = 1/N and N is lines per metre.
- Constructive interference needs a path difference of a whole number of wavelengths; destructive, an odd number of half-wavelengths.
- Sound intensity level in dB: 10 log₁₀(I/I₀), with I₀ = 1 × 10⁻¹² W/m².
Worked example
A grating has 500 lines per mm. Find the angle for the second-order image of light of wavelength 600 nm. d = 1/(500 × 10³) = 2.0 × 10⁻⁶ m. nλ = d sinθ, so sinθ = (2 × 600 × 10⁻⁹)/(2.0 × 10⁻⁶) = 0.60. θ = 37°.
Traps
- Mixing degrees and metres in the grating formula without converting.
- Measuring the angle to the grating surface instead of the normal.
- Claiming frequency changes when light enters glass.
- Confusing diffraction (spreading) with interference (superposition).
- Forgetting that second order means n = 2, not θ doubled.