Answers · Waves in a ripple tank

Waves in a ripple tank — return questions key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. VR1 · Return

Model answer. (a) Sound is longitudinal. Light, like all electromagnetic radiation, is modelled as transverse, and so are water waves and seismic S-waves. (b) The flame moves to and fro because the air around it vibrates to and fro about its own place as the sound passes. The flame is not blown away, so the air is not carried away from the loudspeaker: the sound transfers energy, not the air. (c) A frequency of 425 Hz means that 425 waves pass a point each second. λ = v ÷ f = 340 m/s ÷ 425 Hz = 0.80 m. (d) Push the end of the slinky forwards along its length and pull it back, again and again. The coils bunch up and spread out, and each coil vibrates to and fro parallel to the direction the wave travels, just as the air particles do in a sound wave: both are longitudinal.

A full answer includes

(a) I wrote that sound is longitudinal.
(a) I gave one transverse wave: light (any electromagnetic wave), water waves or seismic S-waves.
(b) I wrote that the air vibrates to and fro about its own place.
(b) I wrote that the sound transfers energy, not the air itself.
(c) I wrote that 425 waves pass a point each second.
(c) I wrote λ = v ÷ f = 340 m/s ÷ 425 Hz.
(c) I wrote 0.80 m.
(c) I did not divide the frequency by the speed.
(d) I wrote: push the end forwards and pull it back, along the length of the slinky.
(d) I explained that the coils then vibrate parallel to the direction of travel.
(d) I wrote that the air particles in sound vibrate the same way.

Also accept

(a) longitudinal; light (or water waves, or S-waves) is transverse (b) the air vibrates about its place: energy, not air, is transferred (c) 425 waves each second; 0.80 m (d) push and pull the end along the slinky: vibration parallel to the travel, like sound

Watch for

“The air next to the loudspeaker is blown across the room.” — The flame flickers to and fro but is not blown away: the air only vibrates about its own place, and the energy travels on.
“λ = f ÷ v = 1.25 m.” — That is upside down. Rearrange v = fλ by dividing both sides by f.

2. VR2 · Return

Model answer. (a) Diffraction: the sound bends round the corner of the building into the yard. (b) The rumble. Its wavelength, about 3 m, is far longer than the hiss’s, about 5 cm, and the longer the wavelength, the further waves bend round an edge. (c) Reflection at a plane surface. The reflected sound is in the same air, so its speed, frequency and wavelength are unchanged; only its direction changes. (d) Lay a single straight barrier in the tank so that the waves pass its end. On the screen, the straight waves bend round the end of the barrier into the water behind it, with curved wavefronts. (e) Change 2. After change 1 the 30 m gap is 30 ÷ 5.0 = 6 wavelengths wide. After change 2 the 40 m gap is 40 ÷ 15 ≈ 2.7 wavelengths wide, much closer to one wavelength, and waves spread out most when the gap is about the same size as the wavelength.

A full answer includes

(a) I wrote diffraction.
(b) Supplement: I wrote that the rumble bends further.
(b) Supplement: I wrote that the rumble’s wavelength is much longer.
(b) Supplement: I wrote that longer waves bend further round an edge.
(b) Supplement: I did not explain it by how loud or how high the sounds are.
(c) I wrote reflection.
(c) I gave one unchanged property: its frequency, its wavelength or its speed.
(d) I wrote: place a single barrier in the tank so that the waves pass its end.
(d) I wrote that the waves bend round its end into the water behind it.
(e) Supplement: I wrote change 2.
(e) Supplement: I wrote that after change 1 the gap is six wavelengths wide.
(e) Supplement: I wrote that after change 2 the gap is less than three wavelengths wide.
(e) Supplement: I wrote that waves spread out most when the gap is about one wavelength wide.
(e) Supplement: I did not choose change 1 just because its gap is narrower.

Also accept

(a) diffraction (b) the rumble: longer wavelength, bends further (c) reflection; frequency, wavelength or speed unchanged (d) a single barrier: the waves bend round its end (e) change 2: the gap is about 2.7 wavelengths wide, against 6

Watch for

“The hiss bends further, because high sounds carry further.” — How far a wave bends round an edge depends on its wavelength. The rumble’s wavelength is far longer, so it bends further.
“Narrowing the gap always spreads the waves out more than any other change.” — Narrowing helps only as far as it brings the gap closer to one wavelength. The 30 m gap is still six wavelengths wide; the longer swell makes the 40 m gap less than three.