Making Waves

Ripple tanks often cost a great deal of money. If your school does not have a ripple tank available for you to use, there's a quick and dirty hack you can set up for very little money using a plastic box frame (about $20 new or often found for less in secondhand shops).

Decide whether you will project the image of the moving waves up on the ceiling or down on the floor.

• If you want to project the image on the ceiling, place the light at least a foot or two beneath the tray, pointing straight up. (Be careful not to hurt yourself as you crane your neck!)
• If you want to project the image onto the floor, suspend the light at least a foot or two above the ripple tank, pointing straight down. Have the students gather around the projection on the floor to look at the results together.
• If you want to project onto the wall, you can use a mirror angled at about 45° to transfer the image. Your school may still have old overhead projectors you can use for this purpose.

See bird's-eye photo and diagram (top row) and side-view diagrams of setup for ceiling (bottom row, left) and floor (bottom row, right).

Pull two desks apart from one another, as far as the longest side of the clear tray you will use. For example, if you have a 18"x24" acrylic frame, pull the desks about 22 inches apart. Lay the clear tray across the gap between the desks. Fill the tray at least 0.25 inches or 5 mm deep.

You may find that vibrations in the floor or the tables easily transfer to the water surface. Consider using some kind of foam to dampen the effects.

Why does this work?

As the light passes through the thin layer of water, some of it is absorbed: more light is absorbed where the water is deeper (as with a wave crest, which will appear as a darker area) than where it is shallower (like in a trough, which will appear as a brighter area). When a wave hits some kind of an obstacle, these water waves react as all waves do.

Bird's-eye view of setup

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Side view of two setups
(projecting onto ceiling, left; projecting onto floor, right)

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Paraphrase:

Have you ever noticed the patterns that pebbles or raindrops make when they land on the surface of a pond or puddle?

Display and discuss video.

Ask students:

• What is the shape of the wave coming away from the dropped pebbles and rocks? Is there a pattern?
• What happens when the patterns intersect?

Using a stiff wire (like a straightened paper clip), the tip of a pen or pencil, a dripping eyedropper, or your finger, tap the surface near the middle of the tank to create several circular ripples in quick succession that radiate out from the point you touch. Try not to touch the bottom of the tank as you do (since this will send energy through the material of the tank and send waves out from the sides of the tank).

Paraphrase:

This is the kind of wave we get when a raindrop falls on a puddle or a pond. What do you see? How would you describe these waves?

As you make the waves, ask students to describe the pulses of energy as it radiates out:

• What is its shape?
• Do the waves slow down or speed up as they get to the edges?
• Does the speed of the wave change if you touch the surface of the water more quickly or more slowly?
• How long does it take for the pulse to travel from where you touched the water to the edge of the ripple tank?
• Do the waves travel in all directions at the same speed? How do you know they do or don't? What would make the speed of the wave change?

Drop a very small floating item onto the surface of the water, such as a pinch of an insoluble packing peanut.

Ask the students:

• Does the floater move with the waves? Or does it stay in basically the same place?

Demonstrate interference by touching the surface of the water in a constant, similar pattern in two places.

Place the rod near one end of the tank. Give it a quick tap or a short, sharp roll evenly towards the middle of the tank and back again. Form a repeating pattern of waves by cycling this motion.

Ask:

• Why do these waves look different than the ones made with the single pulse point?
• Will the speed of the wave be different if you start with the rod a few inches from the edge and pull it back rather than pushing it forward?
• How does the speed of this wave compare to the speed of the radiating wave?
• Do the ripples change as they move away from the rod? If so, how?

Possible answers:

• In the single pulse, the waves radiate out from one point, making concentric circles. With the rod, the waves radiate from a line, making parallel lines.
• No, the speed of the wave will not be different if you pull the rod back rather than pushing it forward. The wave's speed does not depend on the direction of the first pulse.
• The speeds (calculated by measuring the distance across the tank and the time it takes to get there) shouldn't be different for radiating or linear waves.
• The ripples start out wide, but they sharpen as they get farther from the rod.

Students may wonder why the ripples sharpen as they get farther from the rod. The image of the ripples is sharpest when the light beam crosses at an angle closer to 90°. As the angle of light is more acute (near the rod), the image is stretched and a little blurrier.

Optional: Use a vibrating motor to generate continuous waves.

Linear barrier

Place a flat, linear barrier parallel to the shorter side of the tray, but somewhere in the middle of the tray. (We built our barrier out of Lego bricks about 30 studs long altogether. We made sure the underside of the Lego wall was wet so it’d be less likely to float, and we added levels as additional weight to help keep it in place.)

Tap or roll the rod once to make a single linear wave pulse.

Ask:

What happens to the wave after it hits the flat wall?

Tap or roll the rod rhythmically to make a series of linear wave pulses.

Ask:

What happens to the waves after they hit the flat wall?

Students should notice that the waves reflect.

Paraphrase:

This is a reflected wave.

Use a stiff wire, pen or pencil, eyedropper, or your finger to tap the surface near the side of the tank to send a circular ripple to the flat barrier.

Ask:

What happens to the circular ripple when it bounces off a straight wall?

Where does it seem to come from?

Note the location of the imagined starting place with a coin. Try starting two ripples at the same time, one from the imagined starting place of the bounced ripple and one from the original place.

Now, angle the barrier at about 45°. Send another single linear wave pulse to the barrier. Then a circular one. Try several other angles. For each angle and wave form, ask for the students’ observations.

Curved barriers

Use curved barriers in the shapes of parabolas and half-ellipses (including semicircles). If you are building the shapes out of clay, it might help to print out the shapes and place them temporarily under the tray to “trace” the shape with the clay.

Ask:

What do you think will happen when a straight, linear wave hits this curved wall?

Try tapping the water surface from multiple locations. Then use the rod to roll linear waves at the curved barrier.

A fun trick to try

A single linear wave will reflect into a circular wave that converges to a single point. Mark the point with a coin.

Ask:

Is there a way to make straight line waves bounce off a curved wall?

What do you think will happen if we tap the surface of the water just once from this point?

When you tap the surface of the water just once from this point, it should produce linear waves.

The place where you put the coin is the focal point. A single tap to the surface at this point should reflect off the curved barrier as a linear wave.

The curved barrier reflecting the waves acts as a concave mirror would if the waves hitting it were light waves.

Change the distances and angles from which you send linear waves to the curved barrier.

Ask:

Does the point where the waves meet change?

The answer should be no.

Solar cookers, telescopes, satellite television antennas, and flashlights all use curved reflectors.

Take suggestions from your students for other taps and rolls to try, with and without barriers.

Tap the surface in multiple locations at once.

Make other barrier shapes.

Challenge them to advise how you would make patterns like the ones in the bottom image.

Paraphrase:

Now you try it, in the dry land of electronic simulations!

If the linked window doesn't work, direct your students to the original simulation (with ads) at falstad.com/ripple.

From the website:

This is a simulation of a ripple tank. It demonstrates waves in two dimensions, including such wave phenomena as interference, diffraction (single slit, double slit, and so on), refraction, resonance, phased arrays, and the Doppler effect.

To get started with the applet, just go through the items in the Example menu in the upper right. You can also draw on the screen with the mouse. The predefined setups are just starting points; you can modify the sources and walls as you desire.

We recommend settings similar to the ones in the screenshot at right. Specifically,

• Choose Color Scheme 1 or 4.
• Slow down the effects by sliding "Simulation Speed" pretty far to the left.
• Increase the size of the waves (and legibility of the effects) by sliding "Source Frequency" pretty far to the left.
• Reduce the contrast (to make the waves easier to look at without eye strain) by sliding the last slider, "Brightness," left of center.

Google Earth is an interactive piece of software that maps the world in three dimensions by combining data from satellite imagery, aerial photography, and GIS satellites. Within Google Earth you can find images of waves changing their patterns as they enter harbors, move around boats, piers, and jetties, reflect off of shorelines, and so on, very similar to what students have seen in the Ripple Tank and the Wave Simulator.

Display and discuss slides.

Have students open Google Earth and find these locations in space and time. There's a clock icon on the top toolbar that allows you to scroll back through image updates to find the same image. Google Earth periodically updates its maps.)

Note: This part of the activity is inspired by the article "Teaching Waves with Google Earth" by Fabrizio Logiurato in Physics Education 47:1 (page 73).

 

Reflection

Diffraction and reflection of circular waves:
Port Elizabeth, South Africa
date: 11/8/2006 
coordinates: 33°57'19.98”S,25°38'33.05"E

Reflections of a boat wake by an obstacle:
River Thames, London, England 
date: 11/06/2006 
coordinates: 51°29'42.06"N;0°03'37.86"E

Reflections of a boat wake by an obstacle:
River Thames, London, England 
date: 11/06/2006 
coordinates: 51°28'05.40"N;0°15'18.12"E

 

Diffraction

Diffraction produced by a boat:
Cyprus 
date: 7/7/2007 
coordinates: 34°56'27.21”N, 33°39'17.36”E

Diffraction of waves against the end of a protection barrier:
La Grande-Motte, France 
date: 8/21/2006 
coordinates: 43°33'18.71”N, 4°05'20.01”E

Wave diffraction through an opening: 
Alexandria of Egypt 
date: 12/14/2010 
coordinates: 31°12'28.56"N;29°53'34.66"E

Wave diffraction through an opening:
Théoule-sur-Mer, France 
date: 10/26/2006 
coordinates: 43°31'54.86"N;6 °56'59.41"E

Wave diffraction causes circular erosion of the beach: 
Campo di Mare, Italy 
date: 4/18/2010 
coordinates: 40°32'27.33"N;18°04'09.17"E

 

Interference

Interference between wave fronts produced by the wakes of two boats:
River Thames, London, England 
date: 11/06/2006 
coordinates: 51°27'40.79"N;0°16'05.69"E

Interference between circular waves coming from two openings: 
Rimini, Italy 
date: 5/28/2002 
coordinates: 44°05'15.02"N;12°32'26.07"E

Interference from secondary sources:
Chao Phraya River, Bangkok, Thailand 
date: 8/20/2010 
coordinates: 13°36'40.11"N;100°34'46.60"W

 

Refraction

Wave refraction, as wave fronts approach a beach and bend: 
Villaricos, Spain 
date: 4/26/2002 
coordinates: 37°14'04.59"N;1°47'04.79"E

Wave refraction, as wave fronts approach a beach and bend: 
Sardegna, Italy 
8/18/2010 
coordinates: 39°46'12.64"N;8°27'28.82"E

Wave refraction, as wave fronts approach a beach and bend: 
Chichiriviche, Venezuela 
date: 4/27/2006 
coordinates: 10°55'29.88"N;68°15'26.93"W