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School-college Physics Notes: Waves 11. Ripple tank experiments

Introduction to waves: 11. Experiments with water waves in a RIPPLE TANK

Doc Brown's Physics exam study revision notes: Investigating waves using a ripple tank: measuring the frequency, speed and wavelength of water waves in a ripple tank.

11. Experiments with 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.)

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(i) 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.

(ii) 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

See also SOUND - how to measure the wavelength of sound wave

(iii) 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

Keywords, phrases and learning objectives for waves

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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INDEX physics notes: Introducing the properties of waves

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