衍射和干涉的区别是一个困扰很多人的问题。由Eugene Hecht撰写的教科书《Optics》给出了准确、权威的回答。他说：

“There is no significant physical distinction between interference and diffraction. It has, however, become somewhat customary, if not always appropriate, to speak of interference when considering the superposition of only a few waves and diffraction when treating a large number of waves. Even so, one refers to multiple beam interference in one context and diffraction from a grating in another.”

就是说，干涉与衍射没有显著的物理区别。但是在某种程度的习惯上，即使这不总是恰当的，当只考虑几个波的叠加时，我们称之为干涉，当处理大量波的叠加时，我们称之为衍射。即便如此，人们说多个光束的干涉，但是说来自光栅的衍射。