Interference & Superposition of Waves (Edexcel A Level Physics)

• Interference occurs whenever two or more waves combine to produce a resultant wave with a new amplitude
• Superposition literally means to be positioned over something
  • When waves interfere and combine, they do so according to the principle of superposition

• If two wavefronts are travelling towards each other they will combine by superposition and then pass through 
• The wavefronts will emerge unchanged on the other side

• Interference due to superposition can be constructive or destructive
  • Constructive interference happens when the resultant wave has a larger amplitude than any of the individual waves
  • Destructive interference happens when the resultant wave has a smaller amplitude than the individual waves

Coherence

• Interference is only observable if produced by a coherent source
• Waves are said to be coherent if they have:
  • A constant phase difference
  • The same frequency

Coherent waves (on the left) and non-coherent waves (on the right). The abrupt change in phase creates an inconsistent phase difference

• For example, in light, a coherent beam of light contains light waves that are monochromatic and have a constant phase difference
  • Monochromatic light consists of light waves of a single frequency
  • Laser light is an example of a coherent light source
  • Filament lamps produce incoherent light waves