Why does the lightning delay measure only sound?

Light arrives almost instantly, so the delay belongs to the sound.

Why is the frequency unchanged when sound enters water?

The frequency is set by the source, not by the medium it crosses into.

Why is the fundamental wavelength twice the string length?

Both ends are nodes, and neighbouring nodes are half a wavelength apart.

S5 Physics: Waves and Sound Practice Paper 2

2026-11-01

Tutor Prep Guide · Secondary Physics

Paper two warms up with v = f lambda, a 250 Hz period and sound in a vacuum, then times thunder at 2.5 s, halves a sonar echo and moves a wave from air into water, before a fixed string fundamental and a 500-vibration count, ending with the wavelength mix-up.

Quick answer　300 m/s, 0.004 s, 0.68 m, a medium is needed, 850 m, 600 m, 3 m, 2.4 m at 150 Hz, 250 Hz at 150 m/s, and a lower frequency.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10 questions
Suggested time	25 minutes
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S5 Physics: Waves and Sound – Tutor Lesson Plan
Level	S5

02　Questions (10 items)

FoundationA wave has frequency 200 Hz and wavelength 1.5 m. Find its speed.
FoundationWhat is the period of a wave of frequency 250 Hz?
FoundationSound travels at 340 m/s. Find the wavelength of a 500 Hz note.
FoundationCan sound travel through a vacuum?
IntermediateThunder is heard 2.5 s after the lightning is seen (sound speed 340 m/s). How far away did it strike?
IntermediateA sonar sends ultrasound and receives the echo from the seabed 0.8 s later (sound speed in water 1 500 m/s). Find the depth.
IntermediateA 500 Hz sound passes from air (340 m/s) into water (1 500 m/s) with the frequency unchanged. Find the wavelength in water.
ChallengeA string fixed at both ends is 1.2 m long. Find the wavelength of the fundamental, and its frequency if the wave speed is 360 m/s.
ChallengeA source makes 500 vibrations in 2 s and the wavelength is 0.6 m. Find the frequency and the speed.
Error redoA student says a longer wavelength means a higher frequency. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	300 m/s (200 x 1.5)	Wave speed is v = f lambda.
2	0.004 s (1 / 250)	The period is T = 1 / f.
3	0.68 m (340 / 500)	Rearranging v = f lambda gives lambda = v / f.
4	No, sound needs a medium	Sound is a vibration of particles, and a vacuum has none.
5	850 m (340 x 2.5)	Light arrives almost at once, so the delay measures the sound.
6	600 m (1 500 x 0.8 / 2)	The echo travels out and back, so the time covers twice the depth.
7	3 m (1 500 / 500)	The frequency is set by the source, while the wavelength follows the speed.
8	2.4 m and 150 Hz	Both ends are nodes, so the fundamental is half a wave and the wavelength is twice the length.
9	250 Hz and 150 m/s	The frequency is 500 / 2 = 250, so the speed is 250 x 0.6.
10	In one medium the speed is fixed, so a longer wavelength means a lower frequency	With v fixed in v = f lambda, f and lambda are inversely related.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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