Phase Changes on Reflection or Transmission
There are many situations where interference can take place. The extent to which interference occurs depends on the phase difference between the waves that interfere. To simplify thing, if two waves are in phase then constructive interference occurs, and if two waves are out of phase (so that a crest coincides with a trough) then complete destructive interference occurs.
If the phase of a wave changes when it is reflected of transmitted – simultaneously being refracted – then obviously the nature of interference can change. A phase change is the inversion or change of phase by 180 degrees (or) of a wave that takes place at the interface between two media. A phase takes place according to the rules illustrated in the diagrams below.