Ripple Tank Experiments

A ripple tank is a shallow tank of water used to study wave behaviour. A vibrating bar creates plane (straight) waves; a dipping sphere creates circular waves. A lamp above projects wave patterns onto a screen below.

Measuring wave speed

1. Set the motor to a known frequency ($f$).
2. Freeze the wave pattern using a stroboscope.
3. Measure the distance across several wavelengths and divide to find $\lambda$.
4. Calculate: $v = f\lambda$.

Demonstrating reflection

Place a flat barrier at an angle. Waves reflect with the angle of incidence equal to the angle of reflection (measured from the normal).

A curved barrier focuses the waves to a point (concave) or spreads them out (convex).

Demonstrating refraction

Place a glass plate in the tank to create a shallow region. Waves slow down over the shallow region and the wavelength decreases. If the waves arrive at an angle, they change direction (bending toward the normal in the shallow region).

Demonstrating diffraction

Place two barriers with a gap between them. Waves spread out after passing through the gap.

• Gap much wider than wavelength: little spreading.
• Gap approximately equal to wavelength: maximum spreading (semicircular wavefronts).
• Gap much smaller than wavelength: very little energy passes through.

Common errors and how to correct them

• Saying waves speed up in shallow water. Waves slow down in shallow water (and the wavelength decreases).
• Confusing frequency change with speed change. The frequency does NOT change when waves enter a different depth.

How to apply this in an exam

Describe the setup (tank, motor, lamp/stroboscope). State what you observe and how to measure wavelength and frequency. Explain how to change the variable (e.g. gap width for diffraction).