AS Level Physics · Waves. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.
A progressive wave carries energy from one place to another without carrying matter. Each particle of the rope, spring or water only vibrates about its own rest position.
Displacement is how far a point is from its rest position at one moment, and amplitude is the maximum displacement. The period is the time for one oscillation and the wavelength is the distance between two neighbouring points that are in step. Phase difference tells you what fraction of a cycle one point is ahead of another.
An oscilloscope shows the wave as a trace. The time-base gives time per horizontal division, so you can read the period. The y-gain gives volts per vertical division, so you can read the amplitude.
Students treat intensity as proportional to amplitude. It is proportional to amplitude squared, so if the intensity doubles the amplitude rises by a factor of √2, not 2.
Answer: 0.66 m
Try each one, then open the answer.
Answer: C. f = 1/T = 1 ÷ 4.0 × 10−3 = 250 Hz.
Answer: B. One wavelength corresponds to 360°, so λ/4 corresponds to 90° (π/2 rad).
Answer: A. I = P/(4πr2) = 2.0 ÷ (4π × 25) = 6.4 × 10−3 W/m2.
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