Waves · revision map · A-Level 9702, Topic 7
TOPIC 7 · WAVES
CAMBRIDGE INTERNATIONAL AS & A LEVEL PHYSICS 9702 · ONE-PAGE REVISION MAP
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WAVES7
Progressive Waves
A travelling wave carries energy, not matter; the medium only oscillates about a fixed point.
v = f λv = wave speed, f = frequency, λ = wavelength
- Amplitude A, wavelength λ, period T, phase difference.
- f = 1 / T; phase given in degrees or radians.
- Intensity ∝ A² (and ∝ 1/r² from a point source).
Transverse & Longitudinal
The oscillation direction sets the wave type. v = fλ holds for both.
- Transverse: oscillation perpendicular to travel (light, water, waves on a string).
- Longitudinal: oscillation along travel, as compressions and rarefactions (sound).
transverse
longitudinal
Sound is longitudinal, so it cannot be polarised; light is transverse, so it can.
Doppler Effect (moving source)
A moving source bunches the wavefronts ahead and stretches them behind, so the observed frequency shifts.
fo = fs v / (v ± vs)v = wave speed, vs = source speed · minus for approach, plus for recession
- Approaching: waves squashed, observed f rises.
- Receding: waves stretched, observed f falls.
EM Spectrum
Electromagnetic waves are transverse and share a vacuum speed.
c = 3.0 × 10⁸ m s⁻¹- Order: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.
- Visible light spans about 400 to 700 nm.
- Shorter λ means higher f and higher energy.
- In any medium each band still obeys v = fλ.
short λ, high f → long λ, low f
Polarisation
Only transverse waves can be polarised; sound (longitudinal) cannot.
A polariser passes one plane of oscillation; an analyser at angle θ transmits a fraction of the intensity.
I = I₀ cos² θMalus's law: θ between the transmission axes
BUILDS ON · Kinematics · Oscillations · IGCSE Waves
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WAVES
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LEADS TO · Superposition · Quantum physics