Interference of harmonic waves Interference(干涉) the various pattern caused by the superposition of harmonic waves. Coherence(相干 性)：the sources S are coherence when they have same frequency and fixed phase differenceInterference of harmonic waves Interference（干涉） : the various pattern caused by the superposition of harmonic waves. Coherence （相干 性）: the sources are coherence when they have same frequency and fixed phase difference S1 S2