Problem
State whether each statement is true or false: (a) sound waves in air are longitudinal; (b) a transverse wave cannot be polarised; (c) in a standing wave the nodes stay still; (d) the Doppler effect requires the medium to be moving.
Solution
(a) True. In sound in air, particles oscillate in the same direction as propagation (compressions and rarefactions): a longitudinal wave.
(b) False. Polarisation is precisely a property of transverse waves: the oscillation is perpendicular to propagation and can be oriented (e.g. polarised light). It is longitudinal waves that cannot be polarised.
(c) True. The nodes of a standing wave are the points with always-zero displacement: they stay still while the antinodes oscillate with maximum amplitude.
(d) False. The Doppler effect depends on the relative motion of source and/or receiver with respect to the medium; the medium can perfectly well be at rest. It also holds for light, where there is no medium at all.
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Topics: Onde Concepts: Onda armonica