The concept of optical path and the representation of optical path difference in thin film interference

The rigorous mathematical expression for optical path is given.The concepts of geometrical optical path and physical optical path are proposed,and an optional item of half wave loss is introduced in both the expressions of optical path and optical path difference.Using this new group of formulas, the students can get a quite clear way of thinking in the study of thin film interference,and more importantly,the formulas are general and the results are valid for any complicated cases.