What changes when two waves arrive together?
The principle of superposition
For linear waves, the disturbances add at each point. Two aligned crests reinforce one another. A crest and an equal trough can produce zero displacement at that instant. These are constructive and destructive interference.
A pattern, not a disappearance
Most overlapping waves create a mixture of reinforcement and cancellation across space and time. For light, it is the electromagnetic fields that add. Dark regions in an interference pattern do not mean energy has been destroyed; energy is redistributed across the pattern.
Try changing one thing
Use the PhET simulation below to vary the spacing between two sources while keeping their wavelength fixed. Then hold the spacing steady and change the wavelength. Watch how the regions of reinforcement move.
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