Sound and Light
Sound and Light tests sound as a longitudinal pressure wave and light as a transverse electromagnetic wave, including intensity, reflection, refraction, lenses, critical angle and total internal reflection. Calculations often use Snell’s law, the lens formula or intensity ratios.
Core rules
- Sound needs a medium and travels at about 340 m/s in air. Frequency determines pitch; amplitude determines loudness. Intensity is power per unit area, in W/m².
- Sound intensity level in decibels is 10 log₁₀(I/I₀). A 10 dB increase means ten times the intensity.
- Refractive index is n = c/v. Snell’s law is n₁ sin θ₁ = n₂ sin θ₂, with angles measured from the normal.
- Total internal reflection occurs only when light travels from a denser to a less dense medium and the angle of incidence exceeds the critical angle, where sin C = 1/n.
- For a lens, use 1/f = 1/u + 1/v with the sign convention stated on the paper. A converging lens can form real or virtual images.
Worked example
A ray in air strikes glass of refractive index 1.50 at 30° to the normal. Find the angle of refraction: 1.00 sin 30° = 1.50 sin r, so sin r = 0.500/1.50 = 0.333 and r = 19.5°. The ray bends towards the normal because glass is denser. The critical angle is sin C = 1/1.50 = 0.667, so C = 41.8°. The frequency is unchanged; the speed and wavelength are reduced in the glass.
Traps
Measure angles from the normal, not the surface. Use n = c/v, not v/c. When light enters a denser medium it slows and bends towards the normal. Sound cannot travel through a vacuum. Decibels are logarithmic, so doubling intensity does not double dB.