RIPPLE TANK APPARATUS
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Product Description
It consists of a (usually) plastic tray with a transparent bottom which
is usually lit from above using a desk lamp. Some small types can be
placed over an overhead projector so that the waves can be projected
onto a screen for all to see.
Ripple tanks can be used to show all the basis properties of waves
including diffraction, reflection, refraction and interference.
Ripples are usually created by a small electric motor attached to a
strip of wood suspended by springs or elastic bands. The motor has an
off-centre weight attached so when turned on, makes the strip vibrate
and thus create ripples on the surface of the water.
By lighting the waveforms being created, clear peaks and troughs can be
seen. Trial and error is often needed to get a good image of the waves.
Make a note exactly how you get the best results so it can be re-created
the next time.
To begin you need to be able to make consistent plane waves. Lower the
motor assembly until it just touches the surface of the water and turn
on the motor. You should be able to see straight waves being created
which travel across the surface of the tank. These are often known as
plane waves.
Reflection
Waves can be reflected using pieces of straight or bent metal strip.
Place a straight strip in the water and send a few waves towards it. You
should clearly see the waves reflect off the strip obeying the 'law of
reflection'
If you insert a concave strip and send a few waves towards it, the waves
that are reflected should converge at a specific focal point in the
same way as light behaves in a solar furnace.
Refraction
Take a relatively thick piece of glass or transparent plastic and place
it in the path of the plane waves. The glass should still be covered
with water enabling waves to travel over the top if it. Because the
speed of a wave depends upon the depth of the water, this shallow area
slows down the wave. If the piece of glass is placed at an angle to the
oncoming wave, refraction will take place. You should be able to see a
bend in the plane wave just where the glass is placed.
Diffraction
When waves on the sea travel into a harbour with a narrow entrance, they
diffract, making a semi-circular pattern when passing through. This can
be created using two pieces of metal strip, leaving a gap between the
two. The narrower the gap, the more semi-circular the wave will be when
it passes through.
Interference
By adding two small balls to the motor assembly, two waves can be
created from the point these balls make contact with the water surface.
These pretty much identical wave patterns will interfere with each other
creating visible light and dark areas where they merge. You should also
be ablt to see ‘flat’ areas of destructive interference in places where
the waves cancel each other out.
To cut down on unwanted reflections created by the sides of the tank,
some technicians use small pieces of sponge, glued in place round the
edge to absorb waves so they do not reflect back and interfere with
others.
If lit from above, a large piece of white card should be used underneath
to clearly show up the wave patterns. If lit from below, a low voltage
lamp can be placed under the ripple tank to projects waveforms onto the
ceiling which creates a larger viewing area.
Older ripple tanks are prone to leakages where the plastic start to
deteriorate. Epoxy resin can be used to glue these small gaps shut.
There are many virtual ripple tanks available either online or contained
within some science software. These obviously have many advantages over
the conventional ripple tanks for the technician (spilled water, safety
and lugging it around) and are great for revision purposes but if set
up properly, the original ripple tanks are very effective at showing
pupils these basic wave principles in a very hands-on fashion.
Strobe wheels are often included with ripple tanks. They consist of a
spinnable wheel with slits cut from the edge. When rotated at speed and
looked through with one eye, the image being looked appears to flicker.
Waves appear easier to see using this method and standing waves can
often be observed.