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.%给出了光程定义的数学表达式，提出了“几何光程”和“物理光程”的概念，同时在光程及光程差的表达式中引入“半波损失选择项”。采用这样的表述方式，使学生在讨论薄膜干涉时，思路清晰明了，结果（即得到的公式）适合任意情况。
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