Launch practical
About 60 minutes
- Set up the bench with the 12 items of apparatus.
- Take the readings yourself, into the results table you draw first.
- Process your results and plot the graph.
- Answer exam questions on your own results, which you mark against the mark scheme.
Runs in your browser: a 3D bench with 2D close-ups of each instrument for exact readings. Nothing to install and no account.
What you measure, and how
One run takes 17 readings. You take every one yourself, the way you would in class.
- On the 3D bench (Fig. 1) you set out the apparatus and read each instrument where it stands.
- In the 2D close-up (Fig. 2) the same instrument is drawn large, from the same run, for an exact reading. It is also how the practical runs on a device that cannot show 3D.
- In your table you write what you read, to the instrument’s resolution, and the notebook checks it against the instrument.
How to read it. The zero is on one end of the ruler’s shadow. Slide the line to the other end of the shadow and read the rule, to the nearest millimetre.
What it shows
v = f λ
Wave speed = frequency × wavelength. The frequency is how many waves pass a point each second; the wavelength is the distance from one crest to the next. The speed is set by what the wave travels through (the depth of the water, the tension in the string), so a higher frequency means a shorter wavelength.
The graph to expect. As the frequency increases the wavelength decreases: doubling the frequency halves the wavelength, because the speed v = f λ stays the same.
On the AQA 2027 Physics Equations Sheet
Apparatus
| Item | What it is for | Resolution |
|---|---|---|
| Ripple tank with a glass floor | Holds the water the waves travel through. | — |
| Wooden bar on a motor, with its power unit | Makes straight waves at a steady frequency. | — |
| Lamp on a stand above the tank | Shines through the waves to make a pattern of bright lines. | — |
| Large sheet of white card | The screen under the tank that the pattern falls on. | — |
| Two metre rules | Measure distances on the card and along the string. | 1 mm |
| 10 cm plastic ruler | A scale in the tank to find how much the pattern is magnified. | — |
| Phone with a camera | Photographs the pattern, or films it in slow motion. | — |
| Tally counter and stopwatch | Counts the waves passing a mark in 10 s. | — |
| Signal generator and vibration generator | Vibrates the string at a frequency you set and read. | 0.1 Hz |
| String, pulley and bench clamp | The string the waves travel along. | — |
| 100 g hanger and one 100 g slotted mass | Keeps the tension in the string the same. | — |
| Safety goggles | Protect your eyes near the vibrating string. | — |
Method
In our own words, as the Lab runs it. Your school's method sheet may differ in detail.
Which version are you doing?
Boards set this practical differently. Your course setting picks one; you can change it before you start.
Why: Both start with the ripple tank. AQA then measures waves on a stretched string; Edexcel measures sound waves in a metal rod. The exam questions follow your board.
Level the ripple tank
Put a spirit level on the glass floor, along the tank and then across it, and turn the feet until the bubble sits in the middle both ways.
Why: Water waves travel faster in deeper water, so a sloping tank changes the wavelength from one end to the other.
Put the white card under the tank
Lay the large sheet of white card flat on the bench under the glass floor of the tank.
Why: The lamp shines down through the waves: the card is the screen the pattern of bright lines falls on.
Choose the water depth
Choose how deep to fill the tank.
Do: About 5 mm deep (the AQA method).
Why: The speed of water waves depends on the depth, so the depth must be kept the same (a control variable).
Start the waves
Lower the wooden bar until it just touches the water. Switch on the lamp and the motor, then turn the motor down to a low frequency.
Why: The vibrating bar makes straight waves. At a low frequency the waves are long enough to see and measure.
Put a scale in the tank
Lay a 10 cm ruler on the floor of the tank. Its shadow on the card shows how much the pattern is magnified.
Why: Light spreads out from the lamp, so the pattern on the card is bigger than the real waves.
Choose how to hold the pattern still
The bright lines move across the card. Choose how you will measure the distance across them.
Do: Photograph the card with the metre rule in shot.
Why: A moving, blurred line is hard to line up with a mark on the metre rule.
Choose how many waves to measure across
Place the metre rule at right angles to the bright lines.
Do: Across as many waves as possible, then divide.
Why: The error in judging where a line is stays the same size however many waves you measure across.
Choose how to count the waves
You will count the bright lines passing a mark on the card in 10 s, with a tally counter.
Do: Film 10 s in slow motion and count from the video.
Why: Frequency = number of waves ÷ time. At 6–8 waves a second the lines pass very quickly.
Hang the load on the string
Pass the string over the pulley and hang the 100 g hanger with one 100 g slotted mass on its end (200 g in all).
Why: The weight of the load keeps the tension in the string the same throughout (a control variable).
Put on safety gogglesSafety
Wear safety goggles while the string is vibrating.
Why: A string under tension can snap and whip, or the hanger can jump off.
Choose how to measure the loops
Each loop between two nodes (points that hardly move) is half a wavelength.
Do: Across all the loops, from the vibration generator to the pulley.
Why: The nodes are a little blurred, so judging where each one is has an error of about half a centimetre.
Take the readings
Ripple tank: lay a ruler across the tank at the water surface so its shadow is on the card, then measure the distance across as many waves as possible against that shadow (×3) and count the waves passing a point in 10 s (×3); wavelength = distance ÷ number of waves, frequency = waves ÷ 10 s, speed = frequency × wavelength. String: tune the signal generator until the string shows 2, 3, 4, 5 and then 6 loops; each time read the frequency and measure the length across the loops; wavelength = 2 × length ÷ number of loops, speed = frequency × wavelength.
Repeats: Measure across the waves three times, in different places, and count three times, then take the means. A mean reduces the effect of random errors (judging the middle of a line, starting the count a moment late) but not systematic ones (the magnified pattern, counting by eye). On the string, each loop setting is a new measurement of the same wave speed.
Make safe and clear upSafety
Turn the signal generator to zero and switch it off, then lift off the masses and unhook the string. Switch off the motor and the lamp. Drain the ripple tank into the sink and mop up any spills straight away. Put the metre rules, card and phone away, and switch the room lights back on.
While you take readings
Fine-tune for the steadiest loops
When loops appear, turn the fine control slowly up and down until the loops are as big and as still as possible.
Why: The loops start to show a little before the exact frequency at which the string resonates.
Hazards and control measures
| Hazard | What could happen | Control measure |
|---|---|---|
| Water spilt from the ripple tank, near the mains lamp and power units | Slipping on a wet floor; an electric shock | Mop up spills straight away and keep the lamp and power units on the bench, away from the water. |
| A flashing stroboscope | Can trigger a seizure in someone with photosensitive epilepsy | Do not use a strobe: hold the pattern still with a photo or a slow-motion video instead. |
| The string snapping, or the hanger jumping off, while the string vibrates | Eye injury | Wear safety goggles (or sit behind a safety screen) near the vibrating string. |
| Masses falling from the hanger | Injured feet | Keep feet clear of the hanger and use no more than the 200 g needed. |
Variables
- Independent: you change
- Frequency of the vibration generator in Hz
- Dependent: you measure
- Wavelength on the string in m
- Control: you keep the same
- Tension in the string. The same 200 g load on the hanger throughout.
- The string and its length. The same string, between the same vibration generator and pulley.
- Depth of water in the ripple tank. Keep it the same (about 5 mm) and the tank level.
Results from a specimen run
| Object | Length in cm |
|---|---|
| Ruler in the tank | 10.0 |
| Its shadow on the card | 19.1 |
| Trial | Distance across the waves in cm | Wavelength on the card in cm |
|---|---|---|
| 1 | 52.4 | 5.82 |
| 2 | 52.7 | 5.86 |
| 3 | 52.3 | 5.81 |
| Mean | 52.5 | 5.83 |
| Trial | Number of waves in 10 s | Frequency in Hz |
|---|---|---|
| 1 | 74 | 7.4 |
| 2 | 75 | 7.5 |
| 3 | 75 | 7.5 |
| Mean | 74.7 | 7.47 |
| Number of loops | Frequency in Hz | Length across the loops in cm | Wavelength in m | Wave speed in m/s |
|---|---|---|---|---|
| 2 | 39.1 | 86.4 | 0.864 | 33.8 |
| 3 | 58.4 | 87.4 | 0.583 | 34.0 |
| 4 | 77.3 | 87.6 | 0.438 | 33.9 |
| 5 | 96.6 | 87.0 | 0.348 | 33.6 |
| 6 | 117.1 | 86.8 | 0.289 | 33.9 |
Processing the data
- Wavelength on the card
- 5.83 cm
- Wavelength of the water waves
- 0.0305 m
- Frequency of the water waves
- 7.47 Hz
- Speed of the water waves
- 0.23 m/s
- Wavelength on the string (3 loops)
- 0.583 m
- Speed of the waves on the string
- 34.0 m/s
- Shape of the graph
- As the frequency goes up, the wavelength goes down: doubling the frequency halves the wavelength.
| Quantity | Specimen run | Accepted |
|---|---|---|
| Water waves (about 5 mm deep) | 0.23 m/s | 0.23 m/s |
| Waves on the string (√(tension ÷ mass per metre)) | 34.0 m/s | 33.6 m/s |
Across 120 simulated careful groups, wave speed (v) came out between 0.219 and 0.235 m/s (median 0.227). That is the simulation's model, not a promise of what a real class gets.
Common mistakes and the fix
Mistakes to check in this practical, and what to do instead. In a write-up or an exam answer, the mark goes to naming the step and saying what it does to the readings.
Method: Leave it as it is
The water is up to about 1 mm deeper at one end. The waves speed up and slow down along the tank, so the wavelength changes from place to place and your three distances disagree more (a random error).
The fix: Level the ripple tank. Water waves travel faster in deeper water, so a sloping tank changes the wavelength from one end to the other.
Method: About 1 cm deep
Waves travel faster in deeper water: about 25 cm/s instead of 22 cm/s, so each wave is longer. (Short ripples like these are slower than the 31 cm/s of √(gh), which holds only for long waves.) Not a mistake, but your speed will not match classes using 5 mm.
The fix: Choose the water depth. The speed of water waves depends on the depth, so the depth must be kept the same (a control variable).
Method: Measure on the card only
You measure the pattern on the card, which is magnified about 1.6 to 2.4 times by the spreading light. Your wavelength and speed come out too large by that factor (a systematic error).
The fix: Put a scale in the tank. Light spreads out from the lamp, so the pattern on the card is bigger than the real waves.
Exam answer: Leaving out: wave speed = frequency × wavelength, v = f λ (or any correct rearrangement, in words or symbols)
The fix: Wave speed = frequency × wavelength, v = f λ. Find it on the full AQA 2027 Physics Equations Sheet, then apply it.
Exam answer: Wrong row chosen
The fix: Trial 2 (53 waves) is far below the other two, which agree with each other.
Worked exam-style questions
Written for Study Hack in the style of exam questions, on the specimen run above. They are not taken from any exam board's papers. Try each one before you open the worked answer.
- Question 1Wave equation · Knowledge[1 mark]
Write down the equation that links frequency (f), wavelength (λ) and wave speed (v).
Worked answer and how the marks are given
A full-mark answer
wave speed = frequency × wavelength
v = f λMark scheme
- wave speed = frequency × wavelength, v = f λ (or any correct rearrangement, in words or symbols)
Where marks are usually lost
- Missing: wave speed = frequency × wavelength, v = f λ (or any correct rearrangement, in words or symbols). Wave speed = frequency × wavelength, v = f λ. Find it on the full AQA 2027 Physics Equations Sheet, then apply it.
- Question 2Frequency from a count · Applying knowledge[2 marks]
Use Table 1 to calculate the frequency of the water waves.
Do not use the anomalous result.
Frequency = Hz
Worked answer and how the marks are given
A full-mark answer
Mean = (75 + 76) ÷ 2 = 75.5 waves
f = 75.5 ÷ 10
f = 7.55 HzMark scheme
- Mean of the other two counts ÷ 10 s: 75.5 ÷ 10 (method)
- f = 7.55 Hz
- Allow answers from 7.512 to 7.588 Hz (the marker's tolerance; correct rounding of the student's own working is also accepted).
- Allow the unit written as hertz, hz, /s, s-1, per second, waves per second.
- A correct answer with no working scores full marks.
Where marks are usually lost
- Answer 6.8 Hz (no marks). You included the anomalous count, which drags the mean down. Mean of the two that agree = 75.5, so f = 75.5 ÷ 10 = 7.55 Hz.
- Answer 75.5 Hz (gains 1 mark). That is the number of waves in 10 s. Frequency is waves per second: 75.5 ÷ 10 = 7.55 Hz.
- Answer 0.132 Hz (gains 1 mark). That is the time for one wave (the period), 0.13 s. Frequency = number of waves ÷ time = 75.5 ÷ 10 = 7.55 Hz.
- Question 3Describing the method · Knowledge[6 marks]
A student has a ripple tank, a lamp, a motor with a wooden bar, a large sheet of white card, a metre rule and a stopwatch.
Describe how the student could use this apparatus to determine the speed of water waves.
Worked answer and how the marks are given
A full-mark answer
1. Pour water about 5 mm deep into the ripple tank and put white card on the floor under it.
2. Switch on the lamp above the tank and the motor, at a low frequency, so a pattern of bright lines shows on the card.
3. Lay the metre rule across the tank at the water surface, so its shadow is on the card with the pattern. Photograph the pattern with the shadow in shot (or use a slow-motion video).
4. Measure the distance across as many waves as possible on the photo, using the rule’s shadow as the scale (so the magnification cancels), then divide by the number of waves to get the wavelength of the real waves.
5. Count the number of waves passing a point in 10 seconds, then divide by 10 to get the frequency.
6. Repeat the measurements and calculate means.
7. Calculate the wave speed using wave speed = frequency × wavelength.Mark scheme
Indicative content:
- Water about 5 mm deep in the tank; card under the tank; lamp above; motor (bar) switched on at a low frequency
- Freeze the pattern: photograph it (with the rule, or its shadow from the water surface, in shot as a scale) or film it in slow motion
- Measure across as many waves as possible, against a scale at the water surface (the card pattern is magnified)
- Divide the distance by the number of waves to get the wavelength
- Count the number of waves passing a point in a measured time (e.g. 10 s)
- Frequency = number of waves ÷ time
- Wave speed = frequency × wavelength
- Repeat the measurements and calculate means
- Alternative: time one wave crossing a measured distance (the tank); speed = distance ÷ time
Levels of response Level Marks What the answer does 3 5–6 A coherent method that would give valid results: all the important steps are there, in a sensible order. 2 3–4 A method that might not give valid results: most of the important steps are there, but some are missing or out of order. 1 1–2 A method that would not give valid results: some relevant steps are given, but they are not linked into a workable method. In Study Hack a six-mark answer is not given an automatic level: you tick the points your answer makes, then choose the level that fits it.
- No relevant content: 0 marks.
Evaluate
| Source of error | Type | Effect | Improvement |
|---|---|---|---|
| The pattern on the card is magnified by the spreading light from the lamp | Systematic | Every wavelength measured on the card is too large by the same factor (1.6–2.4 here), so the speed is too large too. | Put a ruler of known length in the tank and use its shadow to find the magnification, then divide by it. |
| Counting fast-moving waves by eye | Systematic | Some waves are missed, so every count, the frequency and the speed are too low. | Film the waves in slow motion and count from the video. |
| Measuring across one wave instead of many | Random | The error in judging each end is a large fraction of one short wave, so the wavelengths scatter. | Measure across as many waves as possible and divide by the number of waves. |
| Judging the middle of moving, blurred lines | Random | Each distance can be a few millimetres out either way. | Photograph the pattern with the metre rule in shot and measure on the photo. |
| Water deeper at one end of the tank | Random | The wavelength changes along the tank, so measurements in different places disagree. | Level the tank with a spirit level so the depth is the same everywhere. |
| Stopping as soon as the loops appear on the string | Systematic | Every frequency is a little below the resonance, so the speed is a little low. | Fine-tune the frequency until the loops are as big and as steady as possible. |
| Judging where the blurred nodes on the string are | Random | Each length can be about half a centimetre out at each end. | Measure across all the loops rather than one, so the error is a smaller fraction. |
| Starting and stopping the count a moment early or late | Random | Each count can be a wave or so out either way. | Count for longer (for example 20 s) and repeat the count, then take a mean. |
Repeatable. When you measure across the waves again in a different place, or count again, with the same method and equipment, you get values close to your first ones.
Reproducible. Another student, or a different method (timing one wave across the tank and using speed = distance ÷ time), gets a similar wave speed.
Other exam boards
How this practical lines up with other GCSE boards' required or core practicals. Codes only; check your own specification for the exact activity.
| Board | Practical | Match | What is different |
|---|---|---|---|
| AQA | Required practical 8 | This page | — |
| Edexcel | 4.17 | Similar, with differences | Ripple tank for the fluid plus a metal rod for the solid, and a judgement of which equipment is suitable. |
Print it
- Blank write-up: the nine-step lab book with the correct table headings, ready to print for a real lesson.
- Specimen lab book: this page's run written up in full, with its exam answers ticked against the mark scheme.
- Printable method card: the practical on one index card. Practise with the cards in Study Hack.
- Practical workbook: a full simulated data set to process on paper, with exam questions and answers.
- Teacher and technician pack: apparatus lists, technician notes and marking points.