Waves, Sound and Light
Waves, Sound and Light tests wave terminology, the wave equation, reflection, refraction, diffraction, sound as a longitudinal wave, echoes, and light behaviour including Snell’s law and total internal reflection. You should be able to describe wave diagrams and solve numerical problems.
Core rules and concepts
- Wave speed v = fλ. Frequency is the number of waves per second (Hz); wavelength is the distance between successive crests or compressions.
- Transverse waves oscillate perpendicular to energy transfer; longitudinal waves oscillate parallel to it. Sound is longitudinal and needs a medium.
- Reflection: angle of incidence equals angle of reflection, measured from the normal. Refraction occurs when wave speed changes at a boundary; frequency stays constant.
- Snell’s law: n = sin i/sin r. The critical angle satisfies sin c = 1/n. Total internal reflection occurs when light travels from a denser to a less dense medium and the angle exceeds c.
- Sound speed in air is about 340 m/s. An echo is reflected sound; the distance to the reflecting surface is d = vt/2 because the sound travels there and back.
Worked example
A ship sends a sonar pulse and receives the echo after 0.80 s. The speed of sound in seawater is 1500 m/s. Find the depth of the water. The pulse travels to the seabed and back, so the total distance is vt = 1500 × 0.80 = 1200 m. Depth = 1200/2 = 600 m. Reasoning: the time given is for the round trip, so halve the total distance. Using d = vt without halving would double the answer.
Traps that cost marks
- Forgetting to divide by 2 in echo problems.
- Measuring angles from the surface instead of the normal.
- Thinking frequency changes during refraction; it is wavelength and speed that change.
- Saying sound can travel through a vacuum.
- Mixing centimetres and metres in v = fλ.