# Translations:Fermat’s principle and reflection and refraction/4/en

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In addition, the following relations hold as a consequence of the proposition that a straight line is the shortest distance between two points. Because is any point on the plane *ST*, it follows that *ACB* is the shortest path and therefore the minimum time path from *A* to the plane *ST* to *B*. Now because *PC* is normal to *ST* and is the image of *A*, both the angle of incidence *ACP* and the angle of reflection *PCB* are complements of the angle *DC*. Hence, the angle of incidence is equal to the angle of reflection. This is the law of reflection.