# Activity: Waves in a ripple tank

### Aims

Investigate the characteristics of waves in a ripple tank (real and simulated)
Introduce Huygens construction.
Use Huygens construction to explain reflection. refraction and diffraction.
State Snell's law.
Solve problems related to Snell's law.

### Observing waves (real)

Using the ripple tank you can observe how a water wave propagates reflects, refracts, diffracts and interferes.

Propagation

• Dip your finger into the water and observe the circular wave fronts that propagate across the tank.
• Dip a straight edge into the water and observe the propagation of a plane wave front.
• Use a vibration generator to produce a continuous wave and observe how the wavelength is related to frequency.

Reflection

• Place a straight reflector into the tank and send circular wave pulses towards it by dipping your finger. Observe how the wave reflects.
• Reflect plane wave pulses using a straight edge.
• Try reflecting straight wave fronts off curved reflectors.

Refraction

This one is difficult to see but worth a try. The speed of water waves is related to depth so when a wave crosses the boundary from deep to shallow it slows down causing it to change direction.

• Create a shallow area by placing a piece of clear plastic into the tank. Observe how the wave changes direction when it crosses the boundary.

Diffraction

A wave will spread out when it passes through an opening that is about the same size as its wavelength.

• Create an opening between two obstacles and observe what happens when a wave passes through it.

Interference

When two waves are incident on the same point the amplitudes add.

• Using two dippers make two identical waves a few cm apart. Observe how the waves add in some places and cancel out in others.

### Observing waves (simulation)

It's quite difficult to see some of the wave properties using a real ripple tank, a simulated one is much clearer. Here we will use Paul Falstads Java applet. Ripple Tank.

• Choose "no sources" in the second menu.
• Choose "mouse edit wave" in the second.
• Click the tank with you mouse and watch the wave spread out.
• Make a bigger wave by drawing a blob then releasing the mouse button.
• Make a plane wave by drawing a line.
• Click 3D to get a 3D view, you can't interact with this but if you stop the simulation in 2D > draw > click 3D > start you will see your ripples in 3D.

Reflection

• To make a reflector choose "mouse edit wall".
• Stop the simulation and first draw a reflector then choose mouse edit wave and draw a wave. Run the simulation to observe the reflected wave.
• Clicking "add borders" will make the waves reflect off the side walls.

Refraction

This one is difficult to set up so might be best to use the ready made simulation.

• Choose "refraction" from the setup options.
• Observe how the direction changes as the wave slows down in the second half of the tank.
• Observe the change in wavelength as the wave slows down.

Diffraction

• Make a narrow opening between two barriers.
• Send a plane wave through the opening and observe how it spreads.

Interference

• Choose "Two sources" from the set up options.
• Observe how the waves add and cancel.
• Try varying the separation between the sources.

### Taking measurements

Reflection

Using an onscreen protractor on the photograph below, show that the angle of reflection is the same as the angle of incidence. If the link doesn't work you can get one here.

• To use the protractor click where you want the centre then use the mouse wheel to rotate the axis and the mouse to move the measuring line.

Refraction

Download this video clip to your desktop and use video analysis in loggerpro and the onscreen protractor to show that

Video

This is Snell's law and we will use it again with light.

Interference

Save this image onto your desktop then insert it into a loggerPro file so that you can use the photo analysis tools to show that:

• The difference in path between source A and point P and source B and point P is one wavelength.
• The path difference to point Q is half a wavelength.

### Huygens construction

The properties of a wave can be explained by considering the wave front to be made of an infinite number of small wavelet sources. Each wavelet progresses forwards adding to give the new wave front.

Here you can see how a plane wave front can be made of a large number of wavelets.

• Use the ripple tank simulation to make a plane wave front out of a line of wavelets. To do this simply stop the simulation and draw a line of dots. Try creating a circular wave front.

Reflection

When a wave front hits a reflector each point on the reflector acts like a small wavelet source causing the wave to propagate in the other direction.

• Notice how the wavelets form the reflected wave.

Refraction

When the wave passes into a different medium the wavelets progress at a different velocity causing a change in direction.

• Try varying the angle of incidence and the refractive index (n).

Diffraction

In this animation you can see how the wavelets making up a wave front passing through a small opening spread out forming areas of constructive and destructive interference.

### Waves in the sea.

Sea waves exhibit all the properties of a wave but unless you live by the sea you probably won't have noticed them. Have a look along the coastline of your country (if it has a coastline) in Google Earth and see if you can find examples of reflection, refraction, diffraction and interference. The image is one that I found along the coast of Southern England. (It helps if you know where the surfing beaches are).

Ripple tank simulation (GeoGebra)
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