Waves
Waves is the A-level Physics topic covering transverse and longitudinal waves, superposition, interference, diffraction, standing waves and polarisation. It links the ripple tank demonstrations met at GCSE to quantitative treatments of the Young double-slit experiment and stationary waves on strings, both examined as required practicals.
Method
- Identify whether a wave is transverse (oscillations perpendicular to energy transfer, so it can be polarised) or longitudinal (oscillations parallel to energy transfer, so it cannot be polarised).
- Use v = f x wavelength to relate wave speed, frequency and wavelength, converting all lengths to metres first.
- Apply the principle of superposition: at any point, the resultant displacement is the vector sum of the individual wave displacements.
- For two-source interference, use path difference to predict constructive interference (path difference = a whole number of wavelengths) or destructive interference (path difference = a whole number plus a half wavelength).
- For double-slit interference, use w = (wavelength x D) / s to calculate fringe spacing, being careful with unit conversions.
- For standing waves on a string, count the antinodes to identify the harmonic, and use the relationship between string length and wavelength for that harmonic.
Worked example
Monochromatic light of wavelength 600 nm passes through two slits separated by 0.50 mm, producing an interference pattern on a screen 2.00 m away. Calculate the fringe spacing observed on the screen.
- Convert all quantities to metres: wavelength = 600 x 10^-9 m, slit separation s = 0.50 x 10^-3 m, D = 2.00 m.
- Write down the fringe spacing equation: w = (wavelength x D) / s.
- Substitute the values: w = (600 x 10^-9 x 2.00) / (0.50 x 10^-3).
- Evaluate the numerator: 600 x 10^-9 x 2.00 = 1.20 x 10^-6.
- Divide by the slit separation: w = 1.20 x 10^-6 / 0.50 x 10^-3 = 2.4 x 10^-3 m.
- Final answer: w = 2.4 x 10^-3 m (2.4 mm).
Practice questions
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Q1State whether sound is a transverse or a longitudinal wave.Show answer
Answer: Longitudinal
Q2A wave has a period of 0.025 s. Calculate its frequency.Show answer
Answer: 40 Hz (f = 1/T = 1/0.025)
Q3A wave has frequency 50 Hz and wavelength 0.80 m. Calculate its speed.Show answer
Answer: 40 m/s (v = f x wavelength = 50 x 0.80)
Q4State the condition on path difference for two coherent waves to produce destructive interference.Show answer
Answer: Path difference = (n + 1/2) x wavelength, for any whole number n
Q5Two coherent sound waves of wavelength 0.50 m meet at a point with a path difference of 1.25 m. State whether the interference is constructive or destructive, showing your reasoning.Show answer
Answer: Destructive; 1.25 m / 0.50 m = 2.5 wavelengths, a whole number plus a half wavelength, which is the destructive interference condition.
Q6A stretched string of length 0.60 m vibrates in its second harmonic (two antinodes). Calculate the wavelength of this standing wave.Show answer
Answer: 0.60 m (in the second harmonic, string length = one full wavelength, so wavelength = 0.60 m)
Exam-style questions
Written in the style of a A Level Science exam paper, with a full mark scheme.
A ripple tank dipper vibrates with a period of 0.040 s, producing waves of wavelength 12 mm. Calculate the speed of the water waves, in m/s.
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Two loudspeakers are connected to the same signal generator, emitting coherent sound waves of wavelength 0.85 m. At a point P, the path difference between the sound from the two speakers is 3.40 m. Determine, showing your reasoning, whether a listener at P hears a loud point or a quiet point.
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A vertically polarised beam of microwaves is directed at a receiver, with a metal polarising grille placed between them, initially oriented vertically so the receiver detects a maximum signal. Describe and explain what happens to the signal detected as the grille is slowly rotated through 90 degrees.
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Free printable worksheet
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