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# Studying Wave Properties in a Ripple Tank

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Author: Marc Rosner

Waves are a form of energy derived from the periodic motion of a solid, liquid, or gas. You are probably most familiar with them in water; however, waves can travel through other substances, such as gases and solids. The substance a wave travels through is called the medium. Sound is produced by waves in air. Waves move in straight lines and at constant velocities through uniform mediums. Waves can even travel through a vacuum, in the form of electromagnetic radiation such as light and radio waves.

Sound waves travel approximately 340 m per second through air. Light waves travel nearly a million times faster, at 300 million m per second.

All waves exhibit various properties that you can demonstrate: reflection, refraction, diffraction, and interference.

### Materials

• large, shallow, clear glass pan
• clear glass- or plastic-topped coffee table (or table-size piece of Plexiglas and 2 sawhorses)
• lamp with 100-watt clear incandescent bulb or other bright, concentrated light source
• room with a white ceiling
• water
• metric ruler
• pen or pencil
• stopwatch
• notebook
• wire cutters
• I-m section of rubber tubing (An old garden hose works well.)
• wire coat hanger or other firm wire
• small, rectangular clear plate (Suggested dimensions are 5 by 5 by 0.5 cm. You can ask a hardware store to cut the square from acrylic or glass and have them dull the edges with sandpaper so they do not cut you.)

### Procedure

To Set Up the Ripple Tank

1. Use a shallow glass pan, the bigger the better. Place it on a clear coffee table through which you can see the floor. Alternatively, you can support the pan on a table-size piece of Plexiglas placed across a pair of sawhorses. It is important that the center is unobstructed. Place a bright light under the pan, aimed upward at the ceiling. It is best to choose a room with a plain white ceiling.
2. Check the pan for stability. Add a small quantity of water, to a depth of I to 2 cm.
3. Turn off the room lights. Take a capped pen or a pencil and gently tap the water once, briefly dipping the tip under the surface. Look at the waves in the pan and their reflection on the ceiling. Notice how the waves move outward from the disturbance, travel through the water, and bounce off the sides.

Eventually the wave energy dissipates (disperses or moves away from the source), leaving the surface of the water calm again.

Tap the surface multiple times to create a repeated source of waves for several seconds. Practice both techniques.

You can calculate the velocity of the wave with a stopwatch. Velocity equals distance divided by time. Divide the distance it takes a wave to cross the pan by the time, and you'll have the wave's velocity.

To Demonstrate Reflection

1. Using wire cutters, cut a short 10-to-15-cm length of tube. Cut a piece of firm coat hanger wire to the same length. Pass the wire into the tube. You should be able to bend or straighten the tube. Adjust the tube so that it is straight, 94 then lay it in the center of the ripple tank to create a barrier. The top half of the barrier should be above the surface of the water. Adjust the water level if necessary.
2. Create a wave disturbance 10 cm away from one side of the barrier. Notice how the waves bounce off the barrier at an angle.
3. You have just demonstrated Snell's law: The angle of incidence (angle at which light strikes a surface) is equal to the angle of reflection (angle at which light reflects off a surface). This is written .

4. Bend the barrier into a curve, then put it back in the water. Make a wave disturbance in the middle of the concave side. (A concave curve bends inward—think "cave." A convex curve bends outward.) Notice how the waves bounce back toward a central area or focal point. The waves are said to converge.
If you bend the barrier into a parabola, all the waves will converge on a focal point (For information on parabolas, see chapter 4, "Demonstrating Orbits:')

Now make a disturbance on the convex side. The waves diverge outward from the focal point.

To Demonstrate Refraction

1. Remove the barrier and place a rectangular plate in the pan. If you use glass less than 0.5 cm thick, you may need to stack two or three plates. Press the plate(s) down firmly in the center of the pan.
2. Create a wave disturbance on one side of the pan. Notice that as the waves pass over the plate, they change speed relative to those passing by the sides. When waves enter a new medium (here, shallower water is the new medium), they change speed. This is called refraction.
3. Light waves refract when they go from air to water, or air to glass. Otherwise, how could we see those clear substances at all?

To Demonstrate Diffraction

1. Make another barrier out of tubing as you did in step 1 of the reflection experiment. Place the two barriers in the middle of the pan so that they are end to end separated by a small gap.
2. Send a wave toward the gap. Notice how the waves emerge on the other side out of the gap. Obstacles and small openings diffract (change the direction and intensity of) waves.

References

Making Waves: Finding Out about Rhythmic Motion (Boston Children's Museum Activity Book) by Bernie Zubrowski (New York: Morrow/Avon, 1994).

Waves: The Electromagnetic Universe by Gloria Skurzynski (Washington, D.C.: National Geographic S0ciety. 1996).

Making Waves: www.li.net/-stmarya/stm/home.htm

"Tsunami!" by Frank I. Gonzalez: www.sciam.com/1999/0599issue/0599gonzalez.html