All EM waves travel at the SAME speed in vacuum: c = 3 × 10⁸ m/s.
Sound in air: ~340 m/s at room temperature.
Measuring Wave Speed — Required Practical
REQUIRED PRACTICAL (RP19) — Measure wave speed:
METHOD 1 — Water waves (ripple tank):
Use a stroboscope to 'freeze' waves.
Measure wavelength from the still image.
Count frequency from the vibrating bar setting.
Calculate: v = fλ.
METHOD 2 — Sound waves:
Connect a microphone to an oscilloscope.
Display the waveform — measure period T from the trace.
f = 1/T.
Using two microphones and measuring time delay to find speed.
EXAM SKILL — reading oscilloscope traces:
Time per division (x-axis) → period T → frequency f = 1/T.
Volts per division (y-axis) → amplitude.
⚠️ Common Mistake
Amplitude is the distance from the equilibrium (centre) to the crest — NOT from crest to trough (that's double the amplitude). Frequency and period are reciprocals: f = 1/T. Don't confuse wavelength (one full cycle length) with amplitude (height from centre).
📐 Variables
vWave speed (v) is measured in m/s (m/s)
fFrequency (f) is measured in hertz (Hz)
λWavelength (λ) is measured in metres (m)
TPeriod (T) is measured in seconds (s)
AAmplitude (A) is measured in metres (m)
📐 Key Equations
v = f × λ
T = 1 ÷ f
📌 Key Note
Amplitude: equilibrium to crest (m). Wavelength (λ): crest to crest (m). Frequency (f): waves per second (Hz). Period (T): seconds per wave. T = 1/f. v = fλ. EM waves in vacuum: 3×10⁸ m/s. Sound in air: ~340 m/s. RP19: measure wave speed in ripple tank.
🎯 Matching Activity — Wave Property Definitions
Match each wave property to its definition and unit. — drag the symbols on the right to match the component names on the left.
Amplitude
Drop here
Wavelength (λ)
Drop here
Frequency (f)
Drop here
Period (T)
Drop here
Wave speed (v)
Drop here
Time for one complete wave — seconds; T = 1/f
v = f × λ — distance travelled per second in m/s
Maximum displacement from equilibrium — measured in metres
Distance from one crest to the next — metres
Number of complete waves per second — hertz (Hz)
⚽ FIFA Worked Examples
Wave Equation
A sound wave has frequency 500 Hz and travels at 340 m/s. Calculate its wavelength.
F
v = f × λ, so λ = v ÷ f
I
v = 340 m/s, f = 500 Hz
F
λ = 340 ÷ 500
A
λ = 0.68 m
🧪 Required Practical
🔬 RP19 (Physics) — Measure the speed of waves in a ripple tank using v = fλ. Measure frequency and wavelength of waves. Also: measure speed of sound using microphones and oscilloscope.
Know the method, variables, equipment and how to analyse results.
🎯 Test Yourself
Question 1 of 2
1. A wave has frequency 200 Hz. What is its period?
2. A radio wave has wavelength 3 m. What is its frequency? (speed of EM waves = 3 × 10⁸ m/s)
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