• Interference occurs when waves?overlap?and their resultant displacement is the?sum of the displacement of each wave
• This result is based on the principle of superposition and the resultant waves may be smaller or larger than either of the two individual waves
• Interference of two waves can either be:
  • In?phase, causing?constructive interference.?The peaks and troughs line up on both waves. The resultant wave has double the amplitude
  • In?anti-phase, causing?destructive interference.?The peaks on one wave line up with the troughs of the other. The resultant wave has no amplitude

Waves in superposition can undergo constructive or destructive interference

• At points where the two waves are neither in phase nor in antiphase, the resultant amplitude is somewhere in between the two extremes
• Waves are?coherent?if they have the same?frequency?and?constant phase difference

Coherent v non-coherent wave. The abrupt change in phase creates an inconsistent phase difference

• Coherence is vital in order to produce an observable interference pattern
• Laser light is an example of a coherent light source, whereas filament lamps produce incoherent light waves

ANSWER:?B

• At point X:
  • Both peaks of the waves are overlapping. This is?constructive?interference and rules out options C and D
• At point Y:
  • Both troughs are overlapping and so?constructive?interference occurs there
• At point Z:
  • A peak of one of the waves meets the trough of the other. This is?destructive?interference (Row B)