Properties of Waves: A Ripple Tank (page 2)

Diffraction

1. Generate ripples and observe what happens when they encounter a pencil held vertically in their path.
2. What happens when a larger barrier, such as a glass or beaker, is held in the path of the ripples?

Interference

1. 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.)
2. Observe what happens to the pattern as the waves from the two sources overlap and interact with each other.

Expected Results

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.

The Point

Waves exhibit certain characteristic behavior, including reflection, refraction, diffraction, and interference. These properties are common to all types of waves.

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