Physics • Year 11 • Module 3 • Lesson 1
Wave Motion and Types of Waves
Build HSC Band 5–6 extended-response technique on classifying waves, analysing energy transfer without matter transport, and evaluating experimental designs in real Australian contexts.
1. Data + scenario: comparing wave behaviour in the Sydney Opera House (Band 5–6)
8 marks Band 5–6
Scenario. An audience at the Sydney Opera House Concert Hall experiences two simultaneous signals: (1) sound from the orchestra travelling through air, and (2) LED stage lighting. An engineer records the following data in the table below. A physicist in the audience also places a small accelerometer on the floor; the device detects low-frequency vibrations (~20 Hz) from the bass instruments transmitted through the concrete floor structure.
| Property | Orchestral sound (in air) | Stage LED lighting | Bass vibration (through concrete) |
|---|---|---|---|
| Wave type | ? | ? | ? |
| Medium required? | Yes (air) | No | Yes (concrete) |
| Particle oscillation direction | ? | N/A (fields oscillate) | ? |
| Approximate speed | 343 m s−1 | 3 × 108 m s−1 | ~3000 m s−1 |
| Frequency (approximate) | 440 Hz (A4 note) | 5.5 × 1014 Hz | 20 Hz |
| Wavelength (calculated) | ? | ? | ? |
Illustrative data. Sound speed in air at 20 °C. LED wavelength approximately 545 nm (green). Bass vibration speed in concrete is approximate.
Q1. Analyse and evaluate the data above to classify each wave and complete the missing entries in the table. Then address the following points in a continuous response:
- Classify each of the three wave types (mechanical/electromagnetic; transverse/longitudinal) and justify using the medium and oscillation-direction data.
- Calculate the missing wavelengths using v = fλ, showing working for each.
- Explain why the bass vibration arrives at the back of the 2,679-seat hall faster than the airborne sound from the same bass instrument, with reference to the wave speeds given.
- Evaluate whether the audience member at the back of the hall experiences the three wave types at the same time, using the travel distances and speeds provided (assume the hall is 60 m long).
- State one difference in the way energy is transferred by the orchestral sound compared with the LED light, at the particle or field level.
2. Experimental design — investigating whether a wave transports matter (Band 5–6)
7 marks Band 5–6
Research question. A Year 11 student argues: “When a wave passes through water, the water must move from the source to the far end of the tank, because the ripple clearly travels across.” Design a scientific investigation to determine whether the water molecules themselves travel with the ripple, or whether only the energy is transferred.
Constraints: You have access to a standard ripple tank or a large transparent container of water, a wave generator (vibrating paddle or dropping a stone), fine-grain food dye, small cork stoppers, a ruler, a stopwatch, and a video camera. The investigation must be completed in one laboratory session of 70 minutes.
Q2. Design the investigation and present it in the format below.
- State your hypothesis (testable prediction including independent and dependent variables).
- Identify the independent variable, dependent variable, and at least two controlled variables.
- Describe the procedure in at least four numbered steps, including how you will test whether the water molecules travel with the wave or stay localised.
- Explain what result would falsify your hypothesis.
- State two limitations of your design and one way to improve reliability.
Q1 — Sample Band 6 response (8 marks), annotated
Classification: Orchestral sound in air: mechanical and longitudinal. It requires air as a medium [medium column = Yes], and air molecules oscillate parallel to the direction of sound travel (compressions and rarefactions) [1]. LED stage lighting: electromagnetic and transverse. It requires no medium [No], and the oscillating electric and magnetic fields are perpendicular to the direction of travel [1]. Bass vibration in concrete: mechanical and longitudinal. It requires concrete as a medium [Yes], and concrete particles oscillate parallel to the propagation direction, like sound [1].
Wavelength calculations: Sound: λ = v/f = 343/440 = 0.780 m (78.0 cm). LED light: λ = (3 × 108)/(5.5 × 1014) = 5.45 × 10−7 m = 545 nm. Bass in concrete: λ = 3000/20 = 150 m. Show all working. [1 per calculation, award 1 mark for the three correct values]
Why bass vibration arrives first at the back of the hall: Travel time = distance/speed. For d = 60 m: sound in air t = 60/343 = 0.175 s; bass in concrete t = 60/3000 = 0.020 s. The concrete wave travels ~8.7 times faster than airborne sound. The bass vibration therefore arrives at the back seats approximately 0.155 s (155 ms) earlier than the airborne sound from the same bass instrument. This is why seated audiences sometimes feel the bass through the floor before they hear it as sound [1].
Simultaneous arrival evaluation: LED light: t = 60/(3 × 108) ≈ 2 × 10−7 s — effectively instantaneous. Airborne sound: t ≈ 0.175 s. Bass through concrete: t ≈ 0.020 s. The three waves do NOT arrive simultaneously. LED light arrives essentially instantaneously; bass vibration arrives ~0.155 s before airborne sound; airborne sound arrives last. The audience at the back experiences the waves in the order: light, then floor vibration, then sound [1].
Energy transfer difference: For orchestral sound (mechanical), energy is transferred by the successive compression and rarefaction of air molecules — the kinetic and elastic potential energy passes from one air molecule to the next; the molecules do not travel with the wave. For LED light (electromagnetic), energy is carried by oscillating electric and magnetic fields that generate each other — no material medium is required; photons of energy travel at 3 × 108 m s−1 through the air and through the vacuum of space [1].
Marking criteria summary (8 marks): 1 = classifies all three waves correctly (mechanical/EM and T/L) [3 waves, award 1 if all correct]; 1 = correct wavelength for sound; 1 = correct wavelength for light (correct power of 10); 1 = correct wavelength for bass; 1 = correct calculation and explanation of differential arrival time; 1 = evaluates simultaneous arrival with travel time comparison; 1 = correctly contrasts particle-level energy transfer (sound) vs field-level (light); 1 = uses precise physics terminology throughout (λ, v, f, longitudinal, transverse, electromagnetic, medium).
Q2 — Sample Band 6 response (7 marks), annotated
Hypothesis: If waves transfer energy but not matter, then a cork placed on the water surface will oscillate up and down about its starting position and will not drift horizontally in the direction the wave travels. Independent variable: whether or not a wave is passing beneath the cork. Dependent variable: the horizontal displacement of the cork from its starting position after the wave passes. Controlled variables: wave frequency (keep the paddle speed constant), water depth (maintain at 5 cm throughout), and cork mass and shape (use identical corks). [1 — hypothesis with IV and DV]
Procedure: (1) Fill the transparent container to a depth of 5 cm. Place a small cork on the surface and mark its starting position with a chinagraph pencil on the outside of the container. Begin video recording from directly above. (2) Activate the wave generator (or drop a stone consistently from 10 cm above the surface at one end) to send a series of regular ripples across the surface toward the cork. (3) After 10 wave cycles have passed the cork, pause and record the cork’s new position relative to the starting mark. Measure the horizontal displacement using a ruler. (4) Repeat the observation with a small drop of food dye placed at the same starting position as the cork, and observe using video whether the dye patch drifts forward or merely disperses symmetrically. [1 — four clear steps; 1 — includes a test for whether particles travel forward (dye + position measurement)]
Falsification: If the cork drifts consistently in the direction the wave travels (not merely bobs up and down), the hypothesis would be falsified — it would suggest matter is being transported with the wave. A systematic horizontal drift of the food dye in the wave direction would also falsify the hypothesis. [1]
Limitations: (1) Surface tension effects and any convection currents in the water may cause small horizontal drift of the cork that is unrelated to wave motion, making it harder to isolate the wave effect alone [1]. (2) A ripple tank generates real water waves, which have a small amount of net forward particle motion (Stokes drift) in deep water; the effect may be detectable, blurring the result [1].
Improvement: Repeat the experiment a minimum of five times and calculate the mean horizontal displacement per wave cycle to quantify any systematic drift; use video frame analysis to track the cork position precisely rather than relying on visual inspection [1].
Marking criteria summary (7 marks): 1 = testable hypothesis naming IV and DV; 1 = four steps including a measurement of horizontal displacement; 1 = step uses dye or visual tracking to test whether matter moves forward; 1 = states what would falsify the hypothesis; 1 = first valid limitation; 1 = second valid limitation; 1 = one specific reliability improvement.