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Two dippers, one duck. Where do the waves add up, and where do they cancel?
Wave Interference lab · Last updated: 16 October 2026
Two dippers make ripples in the same tank, and a little duck floats between them. In some places the duck bobs wildly; in others it barely moves. You move the duck around to map where the waves add up and where they cancel.
What you'll discover
- When two waves meet, they add together at every point.
- Crest meeting crest makes a bigger wave; crest meeting trough cancels out.
- Two sources make a fixed pattern of lively and calm lines across the water.
The science in plain words
When two waves overlap, the water at each point simply moves by the total of both waves. This adding up is called superposition, and the patterns it creates are called interference.
Where a crest from one dipper meets a crest from the other, they add to make a crest twice as tall. Trough meeting trough makes a deeper trough. This is constructive interference, and the duck bobs strongly. Where a crest meets a trough, they cancel and the water stays almost flat. This is destructive interference, and the duck sits still.
Whether the waves add or cancel depends on how far each point is from the two dippers. If the two distances differ by a whole number of wavelengths, the waves arrive in step and add. If they differ by half a wavelength, they arrive out of step and cancel. The result is a fan of calm lines spreading out between lively ones.
Interference happens with all waves. Noise-cancelling headphones make sound waves that cancel incoming noise. The colours on a soap bubble come from light waves interfering. In 1801 Thomas Young showed light makes interference patterns, strong evidence that light is a wave.
Where it fits
- India, NCERT: Wave interference goes beyond the NCERT syllabus for classes 6 to 10; the lab builds on waves in Class 9 Chapter 11: Sound.
- United States: Grades 9 to 12
Common questions
What is wave interference?
Interference is what happens when two waves overlap. They add together, making bigger waves where crests line up and cancelling where a crest meets a trough.
How do noise-cancelling headphones work?
A microphone picks up outside noise and the headphones play a matching sound wave that is exactly out of step. Crest meets trough, so much of the noise cancels before it reaches your ear.
Why do soap bubbles show colours?
Light reflects from both the inner and outer surfaces of the thin soap film. The two reflections interfere, strengthening some colours and cancelling others depending on the film's thickness.
Related labs
- Diffraction Patterns: Ripples squeeze through a gap. Do they fan out or go straight?
- Beat Frequency: Two nearly equal notes together go loud, soft, loud, soft. How fast is the wobble?
- Standing Waves: Buzz a cord at just the right rate and it splits into still points and big loops.
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Try it yourself
The Wave Interference lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.