Waves, Sound, and Optics
What this domain tests
Wave properties (frequency, wavelength, amplitude, speed, phase), sound as a longitudinal wave, reflection and refraction of light, lenses and total internal reflection.
Core rules
- v = fλ for all waves; frequency is set by the source and does not change on refraction.
- Transverse waves oscillate perpendicular to travel (light, water ripples); longitudinal waves oscillate parallel (sound).
- Sound needs a medium: about 330–340 m s⁻¹ in air, faster in solids.
- Reflection: angle of incidence = angle of reflection, both measured from the normal.
- Snell's law: n = sin i / sin r; refractive index n = c/v.
- Critical angle: sin C = 1/n; beyond C, total internal reflection occurs (optical fibres, prisms).
- Convex lens: real inverted image when the object is beyond f; virtual magnified image when inside f.
Worked example
Light in water (n = 1.33) meets an air boundary. sin C = 1/1.33 = 0.752, so C ≈ 49°. A ray striking the boundary at 60° to the normal is therefore totally internally reflected and none of it refracts out. Reasoning: because the angle exceeds the critical angle, the refracted ray would require sin r > 1, which is impossible.
Traps that cost marks
- Measuring angles from the surface instead of from the normal.
- Believing frequency changes when light enters a new medium.
- Inverting Snell's law as n = sin r / sin i.
- Drawing a virtual image as though light actually passes through it.
- Assuming sound travels faster in air than in steel.