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Constructive and destructive interference
Beginner
Waves in phase add; in antiphase cancel.
If waves arrive in phase they add (bright, loud); if out of phase by π they cancel. Complete cancellation needs equal amplitudes.
Two speakers produce loud and quiet spots in a room.
Energy isn't destroyed; it's redistributed.
In more detail
Constructive interference occurs when waves are in phase and their amplitudes add. Destructive interference occurs when they are out of phase and cancel.
Real-world uses
- Thin oil films show coloured patterns from constructive and destructive interference.
- Noise-cancelling systems use destructive interference.
- Two loudspeakers in a hall give loud and quiet spots where their waves reinforce or cancel.
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Frequently asked questions
What is the condition for constructive interference?
A path difference of a whole number of wavelengths.
What is the condition for destructive interference?
A path difference of an odd number of half wavelengths.
What is the key condition for constructive interference?
The waves must arrive in phase, so the path difference is a whole number of wavelengths.
Related topics in Waves & Sound
Refraction of waves · Diffraction · Principle of superposition · Path difference and phase · Beats · Standing waves