Unit 2 Section 3 Superposition and Interference


Superposition of Waves

Superposition: The combination of displacements experienced in the instant that two waves pass each other.


  • Principle of superposition states that when two or more waves cross, the resultant displacement equals the vector sum of the individual displacements.
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The superposition of two or more waves can result in interference of: peak-peaks, peak-troughs or trough-troughs. The interference can be: constructive, destructive or total destructive.

Constructive interference: When two waves interfere to make a wave with a larger displacement.

Deconstructive interference: When two waves interfere to make a wave with a reduced displacement.

Total destructive interference: Destructive interference in which the waves completely cancel each other out.


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Phase Difference

Phase difference: The amount by which one wave lags behind another, measured as an angle.

One complete wave is 360 degrees or 2Pie radians

In phase:

  • Two points with a phase difference of zero or a multiple of 360 are in phase.
  • In practice two waves are usually in phase if they come from the same oscillator.
  • Two points on a wave are in phase if they are both at the same point in the wave cycle.
  • Two points in phase interfere constructively with each other.
  • Points in phase have the same displacement and velocity.

Out of phase:

  • Points with a phase difference of odd-number multiples of 180 or 1Pie radians are exactly out of phase.
  • There will nearly always be a phase difference between waves.
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