A-LevelPhysicsWaves9702/21 · May/June 2018 · Q48 Marks

For a progressive wave, state what is meant by (i) the period,

(a)(i)For a progressive wave, state what is meant by (i) the period,[1]
(a)(ii)(ii) the wavelength.[1]
(b)(i)Fig. 4.1 shows the variation with time t of the displacement x of two progressive waves P and Q passing the same point. 4.0 3.0 x/mm 2.0 1.0 wave P 0 0 0/20 0.40 0/60 0.80 t/s -1.0 wave Q -2.0 -3.0 -4.0 Fig. 4.1 The speed of the waves is 20 cm s⁻¹. (i) Calculate the wavelength of the waves. wavelength =[2]
(b)(ii)(ii) Determine the phase difference between the two waves. phase difference[1]
(b)(iii)(iii) Calculate the ratio intensity of wave Q intensity of wave P˙ ratio =[2]
(b)(iv)(iv) The two waves superpose as they pass the same point. Use Fig. 4.1 to determine the resultant displacement at time t = 0.45 s. displacement =[1]

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Mark scheme

Marking points from the official mark scheme (May/June 2018). The mark or mark type is shown in brackets.

[B1] time for one oscillation/one vibration/one cycle or time between adjacent wavefronts/points in phase or shortest time between two wavefronts/points in phase
[B1] distance moved by wavefront/energy during one cycle/oscillation/period (of source) or minimum distance between two wavefronts or distance between two adjacent wavefronts or minimum distance between two points having the same displacement and moving in the same direction
[C1] v = λ/T or v = fλ and f = 1/T
[A1] λ = 20 × 0.60 = 12 cm
[A1] phase difference = 360° × (0.20/0.60) or 360° × (0.40/0.60) = 120° or 240°
[C1] I ∝ A²
[A1] IQ/IP = AQ²/AP² = 2.0²/3.0² = 0.44
[A1] displacement = 1.00 – 3.00 = -2.00 mm

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