Displacement, Coherence, Path Difference, Phase Difference, Superposition

Displacement, Coherence, Path Difference, Phase Difference, Superposition


  • Displacement is a vector quantity that refers to “how far out of place an object is”; it is the object’s overall change in position.
  • It considers the direction and the total distance covered, unlike speed which is a scalar quantity.
  • Displacement is often used in physics calculations and can be represented graphically in motion diagrams.


  • Coherence refers to the consistent phase relationship between waves.
  • Two light sources are perfectly coherent if their phase difference remains constant with time.
  • Laser light is a good example of a coherent light source.

Path Difference

  • Path difference in wave phenomena is the difference in the distance travelled by two waves from their sources to a particular point.
  • In interference patterns, it dictates the type of interference (constructive or destructive) that takes place.
  • Path difference is often measured in terms of wavelength (usually represented by the Greek letter lambda, λ) of the wave. One path difference of ‘λ’ amounts to one whole cycle, or 360° phase difference.

Phase Difference

  • Phase difference is the difference in phase between two waves having the same frequency and reference point.
  • It is an important concept for understanding wave interference and superposition.
  • Phase difference is measured in degrees (°) or radians.


  • The principle of superposition states that when two or more waves meet, the resultant wave is the algebraic sum of the displacements of the individual waves.
  • This principle is the basis for wave interference, and it leads to the formation of standing waves and beats.
  • Superposition is a key principle in the study of waves, applicable to light, sound, and water waves, amongst others.

In wave physics, the concepts of displacement, coherence, path difference, phase difference, and superposition are intimately related and understanding how they interlink is key to grasping the nuances of wave motion and interference. Practice with wave diagrams and mathematics can greatly aid intuitive understanding of these principles.