Physics 11–12 · Year 11

Reflection, refraction, diffraction and superposition in a ripple tank

Module 3: Waves and Thermodynamics (Wave Behaviour)

Practical, model not builtLow risk

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The idea

Water waves change direction at barriers and depth changes, spread through gaps and add where they overlap, and each behaviour follows from the wave's speed and wavelength.

What you need

Ripple tank with lamp, motor-driven straight and twin-point dippers, phone slow-motion video to freeze the pattern
Straight barrier, curved barrier, glass plate to make a shallow region, two barriers to make gaps of 1 cm and 4 cm
Ruler, white paper under the tank, water to about 8 mm depth

How to do it

Reflection: send straight waves at a straight barrier set at 30 degrees; measure the angles of incidence and reflection from the shadows.
Refraction: lay the glass plate to make a shallow region; send straight waves across the boundary at an angle and compare wavelengths and directions on each side.
Diffraction: send straight waves through a 1 cm gap and then a 4 cm gap at the same frequency and sketch the spreading.
Superposition: run the twin-point dipper and count the lines of no disturbance; measure the spacing of adjacent nodal lines at a fixed distance.
Freeze the pattern with slow-motion video and measure the wavelength; compute v = f x lambda.

What you should see

The learner knows the reflection part worked when the angles of reflection and incidence agree within 2 degrees. Waves slow in the shallow region, the wavelength shortens (a shorter wavelength on the shallow side is visible) and the direction of travel bends towards the normal. The narrow gap gives near-circular spreading; the wide gap gives little spreading. Twin sources 3.0 cm apart at a wavelength of 1.0 cm produce nodal lines whose directions satisfy d sin(theta) = (m + 1/2) lambda, so the first nodal line lies at 9.6 degrees.

What changes

What you change: gap width, barrier angle or water depth
What you measure: direction and spread of the transmitted waves
What you keep the same: dipper frequency
water depth in the deep region
amplitude

Common misconceptions

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

Waves refract because they are pushed by the shallow bottom; they refract because their speed changes at the boundary.
Diffraction happens only at small gaps; it happens at every gap and is only large when the gap is about a wavelength wide.

Safety card

Low riskLearners carry it out

Hazards

water near the lamp and mains lead
stroboscope flashing (photosensitive epilepsy)

Controls

low-voltage lamp above the tank, dry hands
use slow-motion video rather than a stroboscope: flashing at 3 to 60 flashes per second can trigger seizures in some people with photosensitive epilepsy (Epilepsy Action), so a stroboscope is used only after checking that no one present is affected

Note

Record the activity in RiskAssess (https://www.riskassess.com.au/) and follow the Science ASSIST risk management information sheet (https://asta.edu.au/resource/ais-risk-management-and-risk-assessment/).

Curriculum references

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

Physics Stage 6 Syllabus (2017), current: Year 11 until the end of 2026, Year 12 until Term 3 2027PH11-10 PH11/12-3
Physics 11-12 Syllabus (2025), not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027, first HSC examination 2028PY-11-02 PY-11WS-03
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities