Waves
6 marks · NormalA sound wave travels through the air and reaches a wall, where it is reflected. The speed of sound in air is 340 m/s.
Draw a labelled diagram to show the reflection of the sound wave from the wall. Label the incident wave, reflected wave, the wall, normal, and angles of incidence and reflection.
If the wavelength of the sound is 0.85 m, calculate the frequency of the wave.
Describe what would happen if the sound wave moved from warm air into cooler air.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Correct diagram showing incident sound wave striking wall, reflected wave, normal (perpendicular to wall), and both angle of incidence and reflection labelled. Both angles must be equal. · must show Incident/reflected waves, wall, normal, angles
(b) Frequency = speed / wavelength = 340 / 0.85 = 400 Hz
(c) Sound waves travel slower in cooler air. As speed decreases when entering cool air, the wave bends (is refracted) towards the normal; direction changes and wavelength decreases.
Must show: Diagram: incident/reflected wave, wall, angles, normal
Common mistakes: Forgetting to label the normal/angles/diagram; Using wrong formula for frequency; Omitting effect of temperature on speed/refraction
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Neat, clear, labelled diagram with all required features (wall, normal, incident, reflected, angles).
B2 (a) · 1 mark · Correctly labels angle of incidence and angle of reflection as equal.
B3 (b) · 1 mark · Correct substitution of wave speed = frequency × wavelength. Shows calculation. (follow-through)
B4 (b) · 1 mark · Accurate evaluation to 400 Hz (accept ±2 Hz for rounding/calculator use). (follow-through)
B5 (c) · 1 mark · States wave slows and direction changes when moving into cooler air (refraction explained).
B6 (c) · 1 mark · Mentions wavelength decreases or bends towards the normal.
Worked solution
a) [Diagram: see below] Draw wall as a vertical line. Incident sound wave approaches the wall, normal drawn at right angles to wall. Reflected wave leaves wall; angle of incidence = angle of reflection. Clearly label incident/reflected, wall, normal, angles.
c) The sound wave enters cooler air, slows down, and bends towards the normal due to decreased speed. Its wavelength decreases but frequency stays the same.