Water waves are probably the most tangible type of wave. For this reason, water waves can be useful in studying wave properties in general. A ripple tank provides a simple, convenient way to produce and study waves and the various types of obstacles they can encounter.
What You Need
- shallow tray or tank with a transparent bottom or commercially available ripple tank
- bright light that can be held or mounted above the tank
- one or several plain sheets of white poster board to serve as a screen on which to view the images produced by the ripple tank
- various props including a straight wall, a curved wall, a thick glass plate about one-half the thickness of the water in the tank, a cup, a pencil, and a manual or mechanical source of ripples
- optional: a way to project the shadows generated, such as an overhead project or a video monitor
Basic wave generation
- Set up the tank with the light overhead. The shadow pattern should be visible on the floor.
- Adjust the height of the light above the tank and the screen below the tank to give the best focus of the shadows from the ripples on the floor.
- Using the tip of a ruler, tap the surface of the water to produce ripples. If you have a vibrating ripple generator, using that might give more consistent results and you won't get tired as quickly from making ripples.
- You should see the ripples spread out in a circular pattern. The tank should be large enough, so this outward moving circular pattern is not obscured by the reflection of the ripples from the side of the tank. Sometimes, a border of foam cushioning is used to minimize side reflections.
- Estimate the wavelength (average distance between ripples) and frequency (number of ripples per second). Estimate the velocity of the ripples. Compare this with the velocity predicted by the wave equation (which applies to all waves): velocity = wavelength × frequency (in cycles per second, which is the same as hertz). If you measure the wavelength in centimeters, the velocity will be in centimeters per second.
- Insert a straight barrier—a wall—in the tank.
- Generate ripples moving toward the barrier at various angles.
- Observe the angle the reflected waves make compared with the incoming waves.
Concave and convex curved reflector
- Insert the concave reflector. This is where the sides curve toward the source of the ripples. Observe how the waves are reflected. Do the waves converge or diverge?
- Generate ripples that originate at that focal point. How do the ripples move?
- Insert (or reshape) the reflector, so it is convex. This is where the sides curve away from the source of the ripples. Do the waves converge or diverge?
- Place a thick plate in the tank.
- What happens to the speed of the waves as the waves cross over the plate? What happens to the wavelength? Does this make sense given that the frequency doesn't change?
- Direct waves to the plate at an angle. What happens when the waves cross from the deep water to the shallower water?
- Generate ripples and observe what happens when they encounter a pencil held vertically in their path.
- What happens when a larger barrier, such as a glass or beaker, is held in the path of the ripples?
- Generate ripples from two different locations. The ripples should be synchronized in such a way that each ripple maker goes up at the same time and down at the same time. (This means the sources of the waves are in phase.)
- Observe what happens to the pattern as the waves from the two sources overlap and interact with each other.
Straight barrier: the incoming angle equals the outgoing angle.
Concave barrier: the reflected waves converge at a focal point.
Concave barrier: ripples generated at the focus regroup and emerge as a single wave.
Convex barrier: waves diverge from any location.
Plate: the waves slow as they cross over the plate; the wavelength increases.
Plate: the waves coming toward the plate at an angle are bent to a less-severe angle.
Diffraction: the wave fronts regroup around a small barrier, but not a larger one.
Interference: two ripple locations result in a fixed pattern of high and low waves.
Why It Works
Water waves exhibit basic wave properties, including:
- Reflection from straight surface: Angle of incidence equals angle of reflection (with all angles defined with respect to the perpendicular or normal line that can be drawn to the reflecting surface).
- Reflection from a concave surface: Waves are reflected from a curved surface with the law of reflection applying to the tangent line of the curve at that point. For approximately parabolic reflectors that include semicircular reflectors, this results in waves passing through a focal point. If the waves are generated at that focal point, they become focused and propagate in a single direction.
- Reflection from a convex surface: Waves diverge and propagate over a wider range of angles than when they started. There is no focal point when waves reflect from a convex surface.
- Refraction: Waves bend toward the perpendicular line (called the normal line) when they enter a region where the light waves move more slowly.
- Diffraction: Waves bend around a barrier in their path if the diameter of that barrier is small compared with the wavelength.
- Interference: Crests and troughs of waves combine to form an overall pattern based on constructive and destructive interference.
Other Things to Try
A large stationary body of water can serve as a large ripple tank. In this case, traveling waves can be observed without the complication of reflections from the side of the ripple tank. Pictured in Figure 65-4 is an interference pattern formed by two rocks thrown into a lake.
Waves exhibit certain characteristic behavior, including reflection, refraction, diffraction, and interference. These properties are common to all types of waves.