Doppler Effect with Water Waves

Materials: ★★☆ Available in most school laboratories or specialist stores
Difficulty: ★★☆ Can be done by science teachers
Safety: ★☆☆ Minimal safety procedures required

Categories: Astronomy and Space, Electromagnetic Spectrum and Waves, Light, Sound

Alternative titles: Water Tank Doppler Demonstration

Summary

The Doppler Effect can be modeled using water waves. When a source of ripples moves through still water, the wavefronts bunch up in front of the moving source and spread out behind, showing how relative motion affects wave frequency.

Procedure

Fill a shallow tray or ripple tank with water.
Create waves using a small object (such as a vibrating dipper, stick, or finger) placed in the water at a fixed point. Observe the circular wavefronts radiating evenly in all directions.
Begin moving the wave source slowly across the water while continuing to make ripples.
Notice that in the direction of motion, the wavefronts are closer together (higher frequency), while behind the motion they are farther apart (lower frequency).
Compare the spacing perpendicular to the motion, which remains unchanged.

Links

Ripple Tank Speed, Frequency, Doppler Effect - Ken Sadowsky:

Variations

Change the speed of the moving wave source to observe how the degree of wave compression changes.
Use two moving sources to show overlapping Doppler patterns.
Record the waves with a camera and analyze spacing of wavefronts using software.
Compare water wave Doppler patterns to sound Doppler shifts with a siren or Doppler ball.

Safety Precautions

Keep electrical equipment (if using a ripple tank with a vibrator) away from water spills.
Wipe up any water splashes to avoid slipping hazards.
Handle glass or plastic tanks carefully to prevent breakage.

Questions to Consider

Why are wavefronts closer together in front of the moving source? (The source is moving into its own waves, compressing them and increasing frequency.)
Why are wavefronts farther apart behind the moving source? (The source moves away from previously emitted waves, stretching them to lower frequency.)
How does this demonstration relate to sound or light? (The same Doppler principle applies to all waves, explaining changes in siren pitch and redshift in astronomy.)
What happens if the wave source moves faster than the waves themselves? (A shockwave forms, similar to a sonic boom.)