Physics · Unit 15

Waves and Sound

Energy that travels as vibrations

A wave transfers energy without transferring matter, and once that is clear the same set of properties describes sound, light, water and earthquakes.

The unit covers wave properties, transverse and longitudinal types, reflection, refraction and diffraction, sound waves specifically, and the Doppler effect and resonance.

This unit breaks down into 19 short steps and 116 questions, starting at difficulty 2 and building to 3. Below you can see exactly what it covers, how the path is structured, and worked examples with explanations.

Steps: 19
Questions: 116
Difficulty: 2-3

What this unit covers

Wave Properties
Reflection, Refraction, Diffraction
Sound Waves
Doppler Effect and Resonance
Transverse and Longitudinal

Where this fits

Needs Light and Sound. Pairs with Light and Optics.

Where people slip

The medium does not travel with the wave. A cork on water bobs and stays put while the wave passes through, and holding onto that picture prevents most wave misconceptions.

How the unit is structured

Waves and Sound runs as 19 short steps that unlock in order. 14 are practice rounds and 5 are challenge rounds that pull together everything before them. Questions start at difficulty 2 and climb to 3 as you progress.

Step 1 · easierStep 19 · harder

Challenge rounds

Example questions

30 real questions from this unit, with the answer and the reason behind it, grouped by what they practise. There are 116 in the unit altogether.

Doppler Effect and Resonance

Build the sentenceLevel 2
1. Arrange the words to complete the rule about resonance.
Answer: resonance occurs at the natural frequency
Resonance occurs when the driving frequency matches an object's natural frequency, producing large vibrations.
Choose all that applyLevel 2
2. Which of these are real examples of the Doppler effect? (Choose all that apply.)
- The changing pitch of a passing car horncorrect
- A police speed radar measuring a car's speedcorrect
- The red-shift of light from galaxies moving awaycorrect
- A mirror forming your reflection
Changing horn pitch, radar speed guns and the red-shift of distant galaxies all use the Doppler effect; a mirror is simple reflection.
Fact or fibLevel 2
3. As an ambulance drives toward you, its siren sounds higher in pitch than after it has passed.
Answer: True
Approaching, the sound waves bunch up so the pitch rises; moving away they stretch out so the pitch drops.
Fill the blankLevel 2
4. Resonance happens when an object is driven at its ____ frequency, causing it to vibrate strongly.
- naturalcorrect
- lowest
- random
- zero
Every object has a natural frequency, and driving it at that frequency causes large-amplitude resonant vibrations.
Multiple choiceLevel 2
5. What does the Doppler effect describe?
- The change in observed frequency when a source and observer move relative to each othercorrect
- The bending of light through a lens
- The loss of a wave's energy as heat
- The splitting of white light into colours
The Doppler effect is the change in the frequency or pitch of a wave heard when the source and listener move closer or farther apart.
Guess the numberLevel 3
6. A siren emits 1000 Hz and moves toward a still observer at 34 m/s (sound speed 340 m/s). Using f' = f v/(v - vs), what frequency in Hz does the observer hear?
Answer: 1111 Hz
f' = 1000 x 340/(340 - 34) = 340000/306 = about 1111 Hz.

Reflection, Refraction, Diffraction

Fill the blankLevel 2
7. The spreading out of waves as they pass through a narrow gap or around an obstacle is called ____.
- diffractioncorrect
- reflection
- refraction
- absorption
Diffraction is the spreading of waves as they pass through gaps or around edges, and it is greatest when the gap is about one wavelength wide.
Match the pairsLevel 2
8. Match each wave behaviour to a correct example.
Answer: Reflection = An echo bouncing off a cliff; Refraction = Light bending as it enters water; Diffraction = Waves spreading through a gap; Absorption = A soft curtain deadening sound
Reflection bounces waves back, refraction bends them into a new medium, diffraction spreads them through gaps, and absorption soaks up their energy.
Multiple choiceLevel 2
9. An echo is produced when sound waves undergo which behaviour?
- Reflection off a surfacecorrect
- Refraction into a new medium
- Diffraction around a corner
- Absorption by soft material
An echo is sound that has been reflected back after bouncing off a hard surface.
Build the sentenceLevel 3
10. Build the diffraction-grating equation from these pieces.
Answer: d sin theta = n lambda
The diffraction-grating equation is d sin theta = n lambda, linking slit spacing, angle, order, and wavelength.
Fact or fibLevel 3
11. Total internal reflection can occur only when light passes from a more optically dense medium into a less dense one, above the critical angle.
Answer: True
Total internal reflection needs the light to go from high to low refractive index and to strike the boundary beyond the critical angle.
Guess the numberLevel 3
12. Light passes from air (n = 1.0) into glass (n = 1.5) at an angle of incidence of 30 degrees. Using Snell's law, what is the angle of refraction in degrees?
Answer: 19.5 degrees
n1 sin(theta1) = n2 sin(theta2), so sin(theta2) = sin(30)/1.5 = 0.333, giving about 19.5 degrees.

Sound Waves

Build the sentenceLevel 2
13. Arrange the words to make a true statement about sound.
Answer: sound cannot travel through a vacuum
Sound is a mechanical wave and cannot travel through a vacuum because there are no particles to carry it.
Fill the blankLevel 2
14. When sound waves reach the ear, they make the ____ vibrate first.
- eardrumcorrect
- cochlea
- lens
- retina
The eardrum is a thin membrane that vibrates when sound waves hit it, starting the hearing process.
Guess the numberLevel 2
15. Roughly how fast does sound travel through air at room temperature?
Answer: 340 m/s
Sound travels through air at about 340 m/s at room temperature, far slower than light.
Multiple choiceLevel 2
16. Why can sound not travel through the vacuum of outer space?
- Sound needs particles in a medium to pass the vibration alongcorrect
- Space is too cold for sound
- Sound is a transverse wave and needs light
- There is too much gravity in space
Sound is a mechanical wave, so it needs a medium of particles to carry the vibrations and cannot travel through a vacuum.
Choose all that applyLevel 3
17. A pipe closed at one end can only produce certain harmonics. Which of these can it produce?
- 1st harmonic (fundamental)correct
- 3rd harmoniccorrect
- 2nd harmonic
- 4th harmonic
A pipe closed at one end supports only the odd harmonics, so the 1st and 3rd are present but the 2nd and 4th are not.
Fact or fibLevel 3
18. When two sound waves of slightly different frequencies overlap, the beat frequency you hear equals the difference between their two frequencies.
Answer: True
Beats arise from superposition, and the beat frequency is the absolute difference |f1 - f2| of the two frequencies.

Transverse and Longitudinal

Choose all that applyLevel 2
19. Which features belong to transverse waves? (Choose all that apply.)
- They have crestscorrect
- They have troughscorrect
- They are made of compressions and rarefactions
- Their vibrations are perpendicular to the direction of travelcorrect
Transverse waves have crests and troughs with vibrations perpendicular to travel; compressions and rarefactions belong to longitudinal waves.
Fill the blankLevel 2
20. A longitudinal wave is made of squeezed-together regions called compressions and stretched-apart regions called ____.
- rarefactionscorrect
- crests
- troughs
- nodes
In a longitudinal wave, rarefactions are the stretched-apart, low-pressure regions between compressions.
Multiple choiceLevel 2
21. In a transverse wave, how do the particles of the medium move compared with the direction the wave travels?
- At right angles (perpendicular) to the direction of travelcorrect
- Back and forth along the direction of travel
- In circles around the source
- They do not move at all
In a transverse wave the medium vibrates at right angles to the direction the wave energy travels.
Odd one outLevel 2
22. Which one of these is NOT a transverse wave?
- Sound in aircorrect
- Light
- A wave on a rope
- A ripple on a pond
Sound is a longitudinal wave; light, rope waves and water ripples are all transverse.
Fact or fibLevel 3
23. P-waves travel faster than S-waves, so they arrive first at a seismograph.
Answer: True
Longitudinal P-waves (primary) are faster than transverse S-waves (secondary), which is why they are recorded first.
Guess the numberLevel 3
24. Radio waves are transverse and travel at 3.0 x 10^8 m/s. What is the wavelength in metres of a 100 MHz FM station? (lambda = c/f)
Answer: 3 m
lambda = c/f = 3.0 x 10^8 / 1.0 x 10^8 = 3.0 m.

Wave Properties

Fact or fibLevel 2
25. If the speed of a wave stays the same, increasing its frequency makes its wavelength shorter.
Answer: True
Because speed = frequency x wavelength, at a constant speed a higher frequency must go with a shorter wavelength.
Fill the blankLevel 2
26. The ____ of a wave is the maximum distance a point moves away from its rest position.
- amplitudecorrect
- wavelength
- frequency
- period
Amplitude is the greatest displacement of the medium from its resting position and is linked to the wave's energy.
Guess the numberLevel 2
27. A wave has a frequency of 5 Hz and a wavelength of 3 m. Using wave speed = frequency x wavelength, what is its speed?
Answer: 15 m/s
Wave speed = f x lambda = 5 Hz x 3 m = 15 m/s.
Match the pairsLevel 2
28. Match each wave quantity to the unit it is measured in.
Answer: Frequency = hertz; Wavelength = metre; Wave speed = metre per second; Period = second
Frequency is in hertz, wavelength in metres, wave speed in metres per second, and period in seconds.
Multiple choiceLevel 2
29. What does the wavelength of a wave measure?
- The distance between two neighbouring points in step, such as crest to crest
- The number of waves passing a point each second
- The greatest distance a point moves from rest
- The time taken for one complete wave to pass
Wavelength is the distance between two corresponding points on a wave, for example from one crest to the next.
Spell itLevel 3
30. The rule that the resultant displacement where waves meet is the sum of their individual displacements is the principle of ____.
Answer: superposition
The principle of superposition states that overlapping waves add their displacements point by point.

Where these questions come from. Each unit starts as a plan of the concepts it should cover and the difficulty it should span. Questions are written against that plan with AI assistance, then checked by a validator that rejects anything without a single defensible answer, an explanation, or plausible wrong options. How we write questions sets out the whole process, and corrections are fixed in the bank and reach the site and the app the same day.

How you practise

This unit mixes 17 different question formats, so you are recalling and applying rather than recognising the same layout every time.

Build the sentence
Choose all that apply
Fact or fib
Fill the blank
Guess the number
Listen and choose
Match the pairs
Multiple choice
Odd one out
Picture question
Put in order
Sequence recall
Sort into groups
Spell it
Tap the pairs
True or false
Type the answer

Practise Waves and Sound

116 questions across 19 steps. Start with step one and crawl at your own pace.

Play this unit

Read about Waves and Sound

Explainers from our blog on what this unit covers. Each one ends with real questions from the bank.

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