Unit 2 Section 3 Superposition and Interference

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.

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.

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.