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A-LEVEL 9702 · AS · TOPIC 7
Waves The chain of the topic: a progressive wave carries energy, not matter; its displacement, period and frequency feed the wave equation v = fλ; the oscillation direction sets the wave type, a moving source shifts the pitch, and visible light is just one band of the electromagnetic family. Around the hexagon are the five ideas; above is what it builds on, below is where it leads.
TOPIC 7: WAVES
CAMBRIDGE A-LEVEL PHYSICS 9702 · PATHWAYS
TheLucidSTEM · thelucidstem.com
BUILDS ON
T2 Kinematics: speed T5 Work, energy, power T17 Oscillations: source motion 7.1
7.2
7.3
7.4-5
TOPIC 7
WAVES
v = f λ
1 · PROGRESSIVE WAVES
Energy travels; the medium only oscillates.
Displacement, amplitude A, wavelength λ, period T.
Frequency f = 1 / T; phase difference in deg or rad.
Intensity ∝ A² (and ∝ 1/r² from a point source).
v = f λ
the wave equation, from speed = distance / time
A
λ
v
displacement against distance, one wavelength shown
2 · TRANSVERSE & LONGITUDINAL
Oscillation direction sets the wave type.
Transverse: oscillation perpendicular to travel
(light, water, waves on a string).
Longitudinal: oscillation along travel, as
compressions and rarefactions (sound).
v = f λ holds for both types
transverse
v
longitudinal
compression
rarefaction
3 · DOPPLER EFFECT (SOUND)
A moving source bunches the wavefronts.
Approaching: waves squashed, observed f rises.
Receding: waves stretched, observed f falls.
v is the wave speed; vₛ is the source speed.
fₒ = fₛ v / (v ± vₛ)
minus for approach, plus for recession
vₛ
higher f
lower f
wavefronts crowd in front of the source
4 · EM SPECTRUM & POLARISATION
Light is transverse, so it can be polarised.
All EM waves travel at c = 3.0 × 10⁸ m s⁻¹ in vacuum.
Radio to gamma; visible light is 400 to 700 nm.
Only transverse waves polarise; sound cannot.
I = I₀ cos² θ
Malus's law: θ between the transmission axes
unpolarised
θ
polariser
analyser
LEADS TO
T8 Superposition: when two waves meet and add T22 Quantum physics: light as photons, E = hf T17 Oscillations: the SHM that drives a wave Each carries one idea forward: a periodic disturbance described by f, λ and v, ready to interfere, to be quantised, or to be driven by SHM.
← Builds on IGCSE: General wave properties