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.)
TOP OF PAGE and
sub-index
(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
measuring speed of a wave
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
INDEX notes: Investigating and introducing
the properties of waves
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.
Use your
mobile phone in 'landscape' mode?
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INDEX notes: Investigating and introducing
the properties of waves
