Wave Interference

Line the two waves up or push them apart, and watch what the combined wave does.

Model note: both waves have the same wavelength and travel the same direction. Those conditions are what let them cancel out completely.

0°
Wave 1
Wave 2
Combined wave Wave 1 + Wave 2
Constructive interference Crests line up with crests, so the combined wave is taller than either one alone.
Learn more about the science

What is interference?

When two waves pass through the same place at the same time, they add together point by point. This adding is called interference. At every spot, you combine the height of Wave 1 with the height of Wave 2 to get the combined wave. Both waves here have the same wavelength and travel the same direction.

Constructive interference

When a crest of one wave lines up with a crest of the other, and troughs line up with troughs, the waves reinforce each other. Their heights add, so the combined wave is taller than either wave alone. In the simulation this happens near 0 degrees.

Destructive interference

When a crest of one wave lines up with a trough of the other, they work against each other. If the two waves are the same height, they cancel out completely and the combined wave flattens to nothing. In the simulation this happens near 180 degrees.

When do the waves cancel out completely?

The simulation shows the cleanest possible case. For two waves to cancel everywhere and leave a flat line, all of these have to be true at once:

  • They have the same wavelength (the same frequency).
  • They travel in the same direction.
  • They are the same height (the same amplitude).
  • They are lined up crest to trough, perfectly opposite.

Change any one of these and the waves still interfere, but they will not flatten out everywhere. Waves with different wavelengths, or waves heading in different directions, add up in some spots and cancel in others instead of cancelling all at once.

The waves are not destroyed

"Destructive" does not mean the waves are gone forever. They only cancel where they overlap. The waves keep traveling and pass right through each other, so they can appear again farther along.

Interference in the real world

When raindrops hit a puddle, each drop sends out its own set of ripples in spreading circles. Where two ripples cross, they interfere just like the waves in the simulation: the water rises higher where two crests meet, and it flattens where a crest meets a trough.

Ripples from many raindrops interfere where they overlap. Because these waves come from many directions and in many sizes, they add and cancel in patches instead of cancelling everywhere, which is why the whole puddle never goes flat at once.

Check Your Understanding

Stuck on a question? Scroll up and use the simulation to figure it out.

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