Learning outcomes
- Describe the main parts and operation of a ripple tank.
- Use a ripple tank to demonstrate reflection, refraction and diffraction.
- Measure wavelength and calculate wave speed.
- Identify limitations and improvements in ripple-tank experiments.
5.1 Apparatus and observation
A ripple tank is a shallow transparent tray of water. A vibrating dipper produces waves, a lamp illuminates the surface, and the pattern is projected onto a screen below. A straight bar makes plane waves; a small point dipper makes circular waves. The water must be shallow enough for a clear pattern but not so shallow that damping is excessive.
A stroboscope flashes at a frequency close to the wave frequency. When the flash timing matches the motion, the wave pattern appears stationary, making wavelength easier to measure. The stroboscope does not stop the waves; it samples them at repeated phases.
5.2 Reflection experiment
Place a straight barrier in the tank at an angle to the incident wavefronts. Observe the reflected wavefronts. Rays drawn perpendicular to the incident and reflected wavefronts obey the law of reflection: the angle of incidence equals the angle of reflection.
To improve the pattern, use long straight wavefronts, keep the barrier vertical, and reduce unwanted reflections from the tank walls using absorbent edges where possible. Mark several wavefronts rather than judging one blurred line.
5.3 Refraction experiment
Place a flat transparent sheet under part of the water to create a shallower region. Waves slow down above the sheet and their wavelength decreases. If they enter at an angle, the wavefronts rotate and the rays bend toward the normal on entering the slower shallow region.
The vibrator frequency is unchanged, so the number of waves produced each second remains constant. Measuring wavelength in deep and shallow regions allows the speed ratio to be found because v is proportional to λ for fixed f.
5.4 Diffraction experiments
Two barriers create a gap. Wide gaps produce nearly straight transmitted wavefronts with curvature near the edges. As the gap is narrowed to about one wavelength, the emerging wavefronts become strongly curved. A single barrier can demonstrate diffraction around an edge.
To investigate wavelength, keep the gap fixed and vary vibrator frequency. In shallow water, changing frequency changes wavelength. Record the patterns systematically and avoid changing water depth at the same time.
5.5 Measuring wave speed
Measure the distance across several consecutive wavefront spacings and divide by the number of wavelengths. For example, the distance from the first to the sixth crest contains five wavelengths. Multiply the mean wavelength by the vibrator frequency using v = fλ.
Major uncertainties include blurred wavefronts, parallax, uncertain frequency, non-uniform depth and counting the wrong number of intervals. Use a ruler on the projected screen, measure many wavelengths, repeat in different positions and quote an average.
Worked examples
Multiple-wavelength method
The distance from crest 1 to crest 7 is 18.0 cm. This contains six wavelengths, so λ = 18.0/6 = 3.00 cm = 0.0300 m. At 12 Hz, v = 12 × 0.0300 = 0.360 m/s.
Practical focus
Investigation
Plan a test of how gap width affects diffraction. Keep water depth, wave frequency and incident wavefront direction constant. Measure gap width in multiples of wavelength and photograph or trace the transmitted pattern for each width.
Examination guidance
- Count intervals, not lines: seven crest lines contain six wavelengths.
- State what is kept constant in an investigation.
- An improvement must be specific, such as measuring five wavelengths and dividing, not simply “repeat”.
Check your understanding
- Why is a stroboscope useful?
- How is a shallow region created?
- What two measurements are needed to calculate wave speed?
Answers
- It makes the moving pattern appear stationary so wavelength can be measured more accurately.
- A transparent plate is placed under part of the water, reducing the depth above it.
- Frequency and wavelength.
