Noise-cancelling headphones
The headphones create a sound wave that cancels unwanted sound near your ear.
Key idea: opposite waves can cancelLabs / Physics / Waves & Interference
Waves & Interference
Watch two sets of ripples overlap and discover why some places become stronger while others become calm.
Why it matters
Where do we use this?
Waves overlap everywhere—from sound and water to light and radio. Engineers use interference to reduce noise, carry information, and measure tiny distances.
Wireless signals
Radio waves can strengthen in one place and weaken in another as they take different paths.
Key idea: path difference mattersMusic and rooms
Reflected sound can make some notes loud in one part of a room and quiet in another.
Key idea: overlap changes loudnessMeasuring precisely
Light interference can reveal changes far smaller than a human hair.
Key idea: patterns reveal small changesOur driving questionWhen do two waves strengthen each other, and when do they cancel?
Explore
Try it first
Move the two sources, change the wavelength, and pause the motion to inspect the pattern.
λ 2.5 cmd 8.0 cmfringes 7
Notice: the sources keep making waves; only their overlap changes the bright and dark pattern.
Investigate
Three guided experiments
Make a prediction before touching the controls. Then test it and explain the evidence in your own words.
Procedure
- Keep wavelength fixed.
- Place the sources close together.
- Move them farther apart and compare.
Record
Count or describe the dark bands.
Explain
What is happening?
Waves add
When waves meet, their displacements add for that moment.
Together gets stronger
Crest meeting crest creates a larger wave. This is constructive interference.
Opposites cancel
A crest meeting a trough can create a calm point. This is destructive interference.
Cancellation
Path difference = (n + ½)λ- λ
- wavelength
- n
- 0, 1, 2, 3…
Worked example
If one wave travels half a wavelength farther, a crest arrives with a trough and the two cancel.
Check your model
Can you predict the pendulum?
Extend
Take it further
Pause the animation and choose one dark line. Trace it across the surface and explain what stays true about the two travel distances.