Wave Investigations
How fast does sound travel through water compared to air? Why does a straw look bent in a glass of water? In this lesson you will design and conduct investigations to measure wave speed in different media, observe reflection and refraction, and learn how to process data, identify trends and draw evidence-based conclusions.
Before You Begin
Imagine you are at a swimming pool and you hear someone diving in at the other end.
Write down your answers before reading on:
- Do you see the splash before you hear the sound? Why might this happen?
- If you are underwater, does sound seem louder or different? What does this tell you about how media affect waves?
- How could you design an experiment to measure how fast sound travels in air versus water?
Know
- That wave speed depends on the medium the wave travels through
- The difference between reflection and refraction
- How to process and represent wave data in tables and graphs
Understand
- Why sound travels faster through solids and liquids than through gases
- How to identify trends and patterns in experimental data
- That conclusions must be supported by evidence from the investigation
Can Do
- Design a fair test to investigate wave properties
- Measure wave speed using appropriate tools and units
- Draw evidence-based conclusions from wave investigation data
Investigating Wave Speed
How the medium affects how fast a wave travels
Waves Motion Synthesis
The speed of a wave is not fixed — it depends on what the wave is travelling through. This makes wave speed an excellent property to investigate experimentally.
Sound speed in different media: Sound travels fastest through solids, slower through liquids, and slowest through gases. This is because particles in solids are packed closely together, so vibrations pass quickly from one particle to the next. In gases, particles are far apart, so the disturbance takes longer to travel.
| Medium | Speed of sound (approx.) | State of matter |
|---|---|---|
| Air (at 20 °C) | 343 m/s | Gas |
| Water | 1 480 m/s | Liquid |
| Steel | 5 960 m/s | Solid |
| Glass | 4 540 m/s | Solid |
Measuring wave speed: For sound in air, one simple method is to measure the time between seeing a distant event (like a hammer hitting a post) and hearing the sound. If you know the distance, wave speed = distance ÷ time. For water waves in a ripple tank, you can measure the distance a crest travels in a known time using a stopwatch and ruler.
Reflection and Refraction
What happens when waves meet boundaries
When a wave encounters a boundary, one of two things typically happens: the wave bounces back (reflection) or it bends and changes speed (refraction).
Reflection: When a wave hits a surface it cannot pass through, it bounces back. Light reflecting off a mirror, sound echoing off a cliff, and water waves bouncing off the wall of a pool are all examples. The angle at which the wave approaches the surface equals the angle at which it leaves — this is called the law of reflection.
Refraction: When a wave passes from one medium into another (for example, light going from air into water), it changes speed. This causes the wave to bend, or refract. A straw in a glass of water looks bent because light from the straw refracts as it moves from water into air. Water waves also refract when they move from deep water to shallow water, which is why waves bend as they approach a beach.
Processing Data and Drawing Conclusions
From raw measurements to scientific insight
Collecting data is only the first step. To make sense of your investigation, you need to process the data and use it to answer your original question.
Step 1 — Organise: Record your measurements in a clearly labelled table with units. Include repeats where possible so you can calculate a mean (average) and spot anomalies.
Step 2 — Process: Perform any calculations needed. For wave speed, divide distance by time. Calculate means from repeated trials. Look for patterns: does wave speed increase as density increases? Is there a consistent ratio?
Step 3 — Represent: Create a graph to visualise trends. Bar graphs work well when comparing categories (e.g., wave speed in different media). Line graphs are useful when one variable changes continuously.
Step 4 — Conclude: Your conclusion should directly answer your investigation question and be supported by your data. Use phrases like "The data shows that..." and "This supports the hypothesis that..." If the data does not support your prediction, that is still a valid result — science advances by testing ideas, not just confirming them.
Common Misconceptions
"Reflection and refraction are the same thing." No — reflection is when a wave bounces back from a surface; refraction is when a wave bends as it passes from one medium into another.
"Sound travels at the same speed everywhere." No — sound speed depends on the medium and its temperature. Sound travels about four times faster through water than through air.
Waves in Australian Research and Life
Great Barrier Reef monitoring: Marine scientists use sound waves (sonar) and light waves (satellite imaging) to monitor coral health. Sound travels quickly through seawater, allowing researchers to map the reef floor, while light-based sensors measure water clarity and temperature from above.
Australian surf forecasting: Agencies like the Bureau of Meteorology use wave data from buoys and satellites to predict surf conditions. Understanding how water waves refract as they approach different coastlines helps predict where waves will break and how large they will be.
Indigenous knowledge of seismic signals: Aboriginal and Torres Strait Islander Peoples have traditionally read signs from the land, including vibrations and tremors. This knowledge reflects a sophisticated understanding that energy transfers through the earth and can signal events at a distance — a concept central to modern wave investigations.
✍ Copy Into Your Books
▾Wave Speed in Different Media
- Wave speed depends on the medium
- Solids > liquids > gases for sound
- v = distance / time or v = f × λ
Reflection and Refraction
- Reflection: wave bounces back from a surface
- Refraction: wave bends when changing medium
- Both are caused by waves meeting boundaries
Processing and Concluding
- Organise data in tables with units
- Calculate means and plot graphs
- Conclusions must be evidence-based
Design a Wave Speed Investigation
Analyse Wave Data
| Material | Speed of sound (m/s) | Density |
|---|---|---|
| Air | 343 | Low |
| Water | 1 480 | Medium |
| Steel | 5 960 | High |
Test Your Understanding
1. In which medium does sound travel the fastest?
2. What happens to a light ray when it passes from air into water?
3. A student measures an echo returning 2 seconds after clapping beside a cliff. If the speed of sound in air is 343 m/s, how far away is the cliff?
4. A student investigates water wave speed in a ripple tank. They keep the water depth the same but change the frequency of the wave generator. Which variable is the student failing to control properly if they want to test how depth affects wave speed?
5. A group of students finds that their measured sound speed is 320 m/s when the accepted value is 343 m/s. Which is the BEST explanation for this difference?
Short Answer Questions
1. Explain why sound travels faster through steel than through air. Refer to particle arrangement in your answer. 4 MARKS
2. A light ray strikes a mirror at an angle of 30° to the normal. Describe what happens to the ray, naming the phenomenon and stating the angle of reflection. Explain why a straw in a glass of water appears bent. 4 MARKS
3. A student conducts an investigation into wave speed in different materials and obtains the following results: air 330 m/s, water 1 450 m/s, glass 4 800 m/s. Explain how the student should process this data and what evidence-based conclusion they should draw. 4 MARKS
Revisit Your Thinking
Go back to your Think First answer. Has your understanding changed?
- Can you now explain why you see a splash before you hear the sound?
- How would you improve your experimental design for measuring sound speed in different media?
Answers
▾MCQ 1
C — Sound travels fastest through steel because the particles in a solid are tightly packed, allowing vibrations to pass quickly from one particle to the next.
MCQ 2
B — When light passes from air into water, it slows down and bends toward the normal line. This bending is called refraction.
MCQ 3
A — The sound travels to the cliff and back in 2 seconds, so the one-way time is 1 second. Distance = speed x time = 343 x 1 = 343 m.
MCQ 4
D — To test how depth affects wave speed, the student must keep all other variables constant, including frequency. Changing frequency would make it impossible to tell whether depth or frequency caused any observed change.
MCQ 5
B — The most likely explanations are that the air temperature was lower than 20 °C (sound speed decreases with temperature) or that there were measurement errors in timing or distance. Both are scientifically valid sources of discrepancy.
Short Answer 1
Model answer: Sound travels faster through steel than air because steel is a solid with particles packed closely together in a fixed pattern. When one particle vibrates, it quickly passes the vibration to neighbouring particles. In air, particles are far apart and move randomly, so it takes longer for the disturbance to travel from particle to particle. The closer packing in solids allows more efficient energy transfer, resulting in a higher wave speed.
Short Answer 2
Model answer: When a light ray strikes a mirror at 30° to the normal, it reflects off the surface at 30° to the normal (law of reflection). This phenomenon is called reflection. A straw in a glass of water appears bent because light from the straw refracts as it passes from water into air. The light slows down when entering the denser water and speeds up when leaving, causing the rays to bend. Our brain assumes light travels in straight lines, so the straw appears to be in a different position than it actually is.
Short Answer 3
Model answer: The student should organise the data in a table with columns for material and wave speed, then plot a bar graph to visualise the comparison. They should check for a clear trend: as the density of the material increases, sound speed increases. The evidence-based conclusion should state: "The data shows that sound travels fastest through glass (4 800 m/s), slower through water (1 450 m/s), and slowest through air (330 m/s). This supports the conclusion that wave speed increases as the density of the medium increases." The student should also note any limitations, such as not controlling temperature.
Wave Jump
Ride the waves and test your knowledge! Jump across platforms, dodge obstacles and answer wave questions to boost your score.
Mark lesson as complete
Tick when you have finished all activities and checked your answers.