Double Slit Experiment with Water Waves

Materials: ★★★ Requires materials not commonly found in school laboratories
Difficulty: ★★☆ Can be done by science teachers
Safety: ★☆☆ Minimal safety procedures required

Categories: Fluids and Surface Tension, Electromagnetic Spectrum and Waves

Alternative titles: Wave Interference with Two Slits

Summary

A tank of water with two slits in a barrier demonstrates how waves interfere. Circular water waves passing through the slits overlap to produce alternating regions of reinforcement and cancellation, creating an interference pattern that illustrates wave behavior.

Procedure

Fill a shallow tank or tray with water and let it settle.
Place a barrier across the tank with two narrow, closely spaced slits.
Generate waves by tapping the water gently with a rod or using a mechanical vibrator in front of the slits.
Observe how the waves spread in circular patterns from each slit.
Watch where the two wavefronts overlap: crests meeting crests form higher waves (constructive interference), while crests meeting troughs cancel out (destructive interference).
To make the pattern clearer, shine a light from above so the interference ripples create a visible pattern on the tank bottom or wall.

Links

Double slit interference of water waves (NCPQ) - Dr Richard Walding:

Physics • Double Slit Experiment - Water Wave Interference Pattern - Points in Time:

📄 Double slit experiment - Anton Paar: https://wiki.anton-paar.com/au-en/double-slit-experiment/

Variations

Try changing the spacing between the two slits and observe how the interference pattern changes.
Use different wave frequencies (faster or slower tapping) to compare the resulting interference fringes.
Compare single-slit versus double-slit setups to highlight the effect of interference.

Safety Precautions

Keep electrical equipment (lamps, vibrators) safely away from water to avoid shock hazards.
Avoid spilling water onto floors to prevent slipping.
Handle glass or acrylic tanks carefully to prevent breakage.

Questions to Consider

What happens at points where two crests meet? (The waves add together, making a taller crest.)
What happens where a crest meets a trough? (They cancel, creating a calm spot.)
How does changing slit spacing affect the interference pattern? (Closer slits spread the pattern wider; farther apart narrows it.)
How does this water-wave experiment help us understand light’s behavior in the double slit experiment? (It shows that light, like water, can behave as a wave, producing interference patterns.)