11.
Experiments with water
waves
in a
RIPPLE TANK
Investigating waves using a ripple tank: measuring the
frequency, speed and wavelength of water waves in a ripple tank.
The humble ripple tank is an excellent way
for students to experimentally investigate waves.
Introduction - common points for all
three wave experiments described
The apparatus consists of:
A transparent tank of water - so you can
clearly observe the waves.
A paddle or dipper is suspended over and into the
water of the ripple tank.
A small
electric motor that is slightly off-set from symmetrical rotation is used to
oscillate the paddle/dipper - this
gives the 'to and fro' rocking action of the paddle sending waves from left
to right.
The vibration generator can have a
built in signal generator so that you can directly set the
frequency of paddle oscillation i.e. frequency of the ripple waves.
A variable power supply allows you to vary the rate
of 'to and fro' oscillation of the paddle that produces the water waves -
this allows a fixed wave frequency to be created AND the ability to vary the
water wave frequency. You might even have a 'sophisticated' frequency
generator to work the humble paddle!
A stopwatch is essential, plus a small
cork, white paper, ruler, pencil, strobe light of variable frequency, graph
paper stuck on card.
(Note: My diagrams are simplified and
don't show the full detail of a good quality ripple tank set-up - could
anybody send me a good image of a really good set-up? properly acknowledged
of course.)
TOP OF PAGE and
sub-index
(a) How to measure the
frequency of a water wave
Here
you place a small cork on the water and start the wave generator to make the
cork bob up and down.
Choose a start point e.g. the top of a bob, start the stopwatch
and time 10 complete oscillations or 'bobs'.
Divide the time by 10 gives you a good average time for
one
period.
Then, a simple calculation ...
Frequency = 1 / period = ? Hz.
You can of course do the calculation directly: Frequency (Hz) =
10 / total time (s)
Repeat with different frequencies.
2nd method
You can measure the frequency by
using a lamp shining down on the ripple tank and projecting shadows onto
a screen under the tank.
Mark a point on the middle of the
screen and switch on the wave generator.
Then count the number of waves
passing in a given time e.g. X waves in 20 seconds using a stopwatch.
Therefore the frequency is
X/20 Hz.
(b) How to measure the
wavelength of a water wave
For this experiment you need a graph card at the back of the
tank.
Set the oscillator going to give a steady stream of water waves.
Direct the strobe light onto the ripple tank so several of the
waves are illuminated.
Adjust the frequency of the strobe light until the waves seem to
stand still.
At this point you can then measure the length of e.g. 10 waves
against the back projection of the graph card.
Just divide the total length by the number of waves to get an
average wavelength in cm ==> m.
You can repeat with different frequencies of the oscillating
paddle.
For variation on the method of measuring
wavelength see
measuring speed of a wave
See also
SOUND - how to measure the wavelength of
sound wave
(c)
How to measure the
speed of a water wave
1st method to
measure the speed of water ripples
You
need two students for this experiment.
Place a large piece of white paper by the side of the tank.
Place a ruler on the paper alongside the tank.
Start the waves going and track the path of a crest on the paper
with the pencil.
The second student starts the stopwatch as the first person
starts marking with the tracking pencil.
Therefore you have time and distance data to calculate the speed
of the waves.
speed (m/s) = distance (m) / time (s)
Don't forget if the distance was measured in cm, divide by 100 to get metres!
In all these experiments where you are changing a variable,
keep all the equipment the same and do three repeats to get a good average.
For a fair test also watch for the variables you are not
testing for ..
e.g. keep the amplitude of the wave constant, keep the
'dipper' in the same position - immerse in same depth of water, use the same
depth of water in the tank.
2nd
method to measure the speed of water ripples
(also indirectly, the wavelength too)
You use the same apparatus set-up
described above, but coupled with a signal generator and employing
the use of a stroboscope and a screen below the tank.
By using a signal generator attached
to the paddle of the ripple tank, you produce water waves of a particular frequency.
The lights are dimmed and turn on the
strobe light to see the wave pattern.
You alter the strobe light frequency
until waves seem to freeze on the screen under the tank - this because
the strobe light frequency matches the water wave frequency and al the
waves are lit at the same point in their cycle at the same time.
You measure the width of 10 crests
(or troughs) from the shadow lines and divide by 10 giving you the
length of one wavelength - you can place a ruler down on the screen
aligned with the direction of wave movement.
You now know the frequency (Hz) and
wavelength (convert to m) of the water waves and so ...
speed = frequency x wavelength =
v = f x
λ = ? m/s
Variation on the above method with
using a strobe light
By using a signal generator attached
to the paddle of the ripple tank, you produce water waves of a particular frequency.
The lights are dimmed and broad
bright light is shone from above the tank to cast a shadow on the screen
below.
Again, you measure the width of 10
crests (or troughs) from the shadow lines and divide by 10 giving you
the length of one wavelength - you can place a ruler down on the screen
aligned with the direction of wave movement.
You now know the frequency (Hz) and
wavelength (convert to m) of the water waves and so ...
speed = frequency x wavelength =
v = f x
λ = ? m/s
INDEX notes: Investigating and introducing
the properties of waves
Key points Introduction to waves:
Ripple
tank experiments
Information
sources for Doc Brown's key points IGCSE-GCSE physics are based on
textbooks & syllabus-specifications for students taking the UK AQA, Edexcel,
OCR 21st Century Science, OCR Gateway science suite, WJEC, CCEA and CIE GCSE
physics 9-1 level science examinations
A structured set of summary revision notes on
ripple tank experiments
to illustrate reflection, refraction, and diffraction,
aligned with the core requirements across major UK GCSE/IGCSE exam
boards (WJEC, CCEA, CIE, AQA, Edexcel, OCR). These experiments help
students visualise wave behaviour at boundaries and obstacles -
perfect for mastering wave interactions.
Ripple Tank Experiments: Reflection,
Refraction & Diffraction
Ripple Tank
Equipment Overview
- Ripple tank with transparent base
- Vibrating bar or point source
- Light source and screen (to project
wave patterns)
- Barriers, glass blocks, and slits
- Ruler and protractor for measurements
1.
Ripple Tank Reflection
Experiment
Setup
- Place a straight barrier in the tank
- Generate plane waves toward the
barrier
Observations
- Waves bounce off the barrier
- Angle of incidence = angle of
reflection
- Use protractor to measure angles from
the normal
Key Concept
- Reflection follows the
law of reflection
- Works for all wave types: sound,
light, water
2.
Ripple Tank Refraction Experiment
Setup
- Place a glass block
or raised platform in the tank to create shallow water
- Generate plane waves toward the
boundary
Observations
- Waves change direction
at the boundary
- Speed and wavelength change,
but frequency stays
constant
- Waves bend towards the normal
when entering shallower (denser) water
Key Concept
- Refraction occurs due to
change in wave speed across media
3.
Ripple Tank Diffraction
Experiment
Setup
- Place two barriers with a gap
between them
- Generate plane waves toward the gap
Observations
- Waves spread out
after passing through the gap
- Maximum diffraction
when gap ≈ wavelength
Key Concept
- Diffraction is more noticeable with
longer wavelengths or
narrower gaps
Typical Exam Board Coverage of describing
and interpreting ripple tank experiments
| Demonstrations of
Reflection
Refraction
Diffraction |
Required Practical |
| Ripple tank |
| Wave properties |
| Ripple tank |
| Wave behaviour |
| Water wave study |
| Wavefront analysis |
Student Tips about describing and
interpreting ripple tank experiments
- Use wavefront diagrams
to visualise wave behaviour
- Measure angles from the normal,
not the surface
- Practise identifying wave interactions
in real-world contexts
- Link ripple tank observations to
light, sound, and EM waves
- Record multiple measurements for
accuracy and reliability
Common Misconceptions involving ripple
tank experiment observations
- “Refraction changes
frequency.”
Frequency stays the same; speed and wavelength change.
- “Diffraction only happens
with sound.”
All waves can diffract - light, water,
and radio included.
- “Reflection only occurs with
mirrors.”
All waves reflect - sound, water, and
light.
- “Waves always bend at
boundaries.”
Only if they hit the boundary at an angle.
For visual reinforcement, you might
enjoy
this ripple tank
video demonstration showing wavefront models of reflection,
refraction, and diffraction in action.
Keywords, phrases and learning objectives for
wave
experiments with ripple tanks
Be able to describe, interpret and explain experiments with water waves in a
ripple tank.
Know how to make
measurements or calculations from observations of the frequency, speed
or wavelength of the water waves.
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Revision notes on describing wave experiments with ripple tanks,
observations & deductions b
ased on the syllabus-specifications
for students taking IGCSE/GCSE level physics examinations, summary
revision notes and key points on describing wave experiments with ripple
tanks, observations & deductions for students taking the AQA
igcse/gcse physics notes on describing wave experiments with ripple tanks,
observations & deductions, Edexcel gcse
physics notes on describing wave experiments with ripple tanks,
observations & deductions, OCR 21st century GCSE
physics notes on describing wave experiments with ripple tanks,
observations & deductions, OCR gateway
GCSE physics notes on describing wave experiments with ripple tanks,
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INDEX notes: Investigating and introducing
the properties of waves
