Two sources are coherent if they have:

  1. the same frequency and a constant phase difference
  2. different frequencies
  3. the same amplitude only
  4. random phases
Answer

A) the same frequency and a constant phase difference

Needed for a stable interference pattern. SLO P-11-D-14 · Understanding

Physics MCQ · More Superposition, Stationary Waves and Beats MCQs · Practise