Ripple tank: reflection at a barrier and refraction into shallow water

The idea

Water waves reflect from a barrier at the same angle they arrive and change speed and direction when they cross into shallower water, the same behaviours that light shows at mirrors and glass.

What you need

• ripple tank with lamp, motor-driven straight vibrator and screen, 1
• straight barrier and concave curved barrier, 1 each
• clear acrylic sheet about 5 mm thick to make a shallow region, 1
• wooden dowel to make single pulses, 1
• ruler, 1
• phone camera for slow-motion stills, 1
• paper towel, mop and a low-voltage (12 V) lamp supply

How to do it

1. Level the tank and fill to 10 mm depth. Dip the dowel once to send a straight pulse; watch it cross the screen and reflect from the far wall.
2. Place the straight barrier at 45 degrees to the pulse direction and send pulses; trace the incoming and reflected wavefronts on the screen and measure the angles to the barrier.
3. Replace with the concave barrier and send straight pulses; note where the reflected ripples converge.
4. Predict whether the ripples will be closer together or further apart over shallow water, and give a reason.
5. Lay the acrylic sheet so its edge is parallel to the wavefronts, leaving about 5 mm of water over it. Run the vibrator at a steady low frequency and photograph the pattern; measure the spacing of ripples over deep and shallow water.
6. Turn the sheet so its edge is at 30 degrees to the wavefronts; photograph and mark the change in direction of the ripples as they cross the edge.
7. Time a single pulse across 30 cm of deep water and across 30 cm of shallow water with slow-motion video; compare speeds.

What you should see

Reflected pulses leave the straight barrier at the angle they arrived, within the precision of tracing. Straight pulses reflected from the concave barrier converge to a bright focus. Waves are slower and closer together over the shallow region, and ripples crossing the edge at an angle bend toward the normal. For long single pulses the shallow-water speed sqrt(g h) is 0.313 m/s in 10 mm and 0.221 m/s in 5 mm, a ratio of 0.71. The vibrator's ripples are only a few centimetres long, and for them the full water-wave relation gives a smaller change: at 4 Hz the wavelength is 72 mm over 10 mm of water and 55 mm over 5 mm (ratio 0.76), and at 8 Hz 31 mm and 28 mm (ratio 0.89), so a low vibrator frequency shows the change most clearly. The learner knows it worked when the ripple spacing is visibly tighter over the acrylic sheet.

What changes

What you change

water depth (deep against shallow region) or barrier angle

What you measure

wave speed and wavelength (from ripple spacing), angle of reflection or refraction

What you keep the same

• same vibrator frequency
• tank level
• same pulse size

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

• Waves speed up in shallow water because there is less water to move.
• Frequency changes when a wave enters a new medium.
• Reflection only happens with light.

Safety card

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

• Science 7–10 Syllabus (2023)SC5-WAM-01SC5-WS-01SC5-WS-02SC5-WS-06
• Australian Curriculum v9AC9S9U04AC9S9I01AC9S9I05

Sources

The pages the author read to write this activity.

1. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/outcomes
2. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/content/stage-5/faafd3c3df
3. spark.iop.org/collections/basic-experiments-ripple-tanks
4. spark.iop.org/refraction-ripples-entering-shallow-water
5. spark.iop.org/reflection-ripples-circular-barrier