Waves
Types of Waves
Transverse waves: oscillations are perpendicular to the direction of wave travel.
- Examples: light, all EM waves, water waves, waves on a string.
- Can be polarised.
Longitudinal waves: oscillations are parallel to the direction of wave travel.
- Examples: sound waves, ultrasound, P-waves (seismic).
- Consist of compressions and rarefactions.
Key Wave Quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Wave speed | v | m/s |
| Frequency | f | Hz (hertz) |
| Wavelength | λ (lambda) | m |
| Period | T | s |
| Amplitude | A | m |
Wave equation: v = fλ
Period–frequency relationship: T = 1/f
Amplitude is the maximum displacement from the equilibrium position. It is related to the energy carried by the wave — greater amplitude = more energy.
Reflection
The angle of incidence equals the angle of reflection (both measured from the normal):
angle of incidence = angle of reflection
A normal is a line perpendicular to the reflecting surface at the point of incidence.
Refraction
When a wave crosses a boundary between two media, its speed changes, causing a change in direction (except at 90° to the boundary).
- Light entering a denser medium (e.g. air → glass): slows down, bends towards the normal.
- Light leaving a denser medium (glass → air): speeds up, bends away from the normal.
Refractive index: n = sin(i)/sin(r) = c/v (where c = speed of light in vacuum, v = speed in medium).
Sound Waves
- Mechanical waves — require a medium (cannot travel through a vacuum).
- Speed in air ≈ 340 m/s; faster in liquids and solids.
- Frequency range for human hearing: 20 Hz – 20,000 Hz.
- Ultrasound: f > 20,000 Hz (above human hearing range).
Ultrasound applications:
- Medical imaging (foetal scans) — pulses reflect from tissue boundaries; time delay → distance.
- Industrial: detect cracks in metals (non-destructive testing).
- Distance = speed × time / 2 (divide by 2 because sound travels there and back).
Core Practical 4 — Investigating waves
Experiment A: Ripple tank (water waves)
- Observe transverse wave properties; measure wavelength directly; vary frequency and observe wavelength change at constant wave speed.
Experiment B: Sound waves using oscilloscope
- Connect microphone to oscilloscope. Observe waveforms for different notes.
- Measure period T from the oscilloscope screen → f = 1/T.
- Louder sound = greater amplitude on screen.
- Higher pitch = higher frequency = closer peaks on screen.
Edexcel examiner tip: know how to read an oscilloscope trace — each horizontal division represents a time interval (set by the timebase); each vertical division represents voltage (proportional to amplitude).
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