Translations:Fermat’s principle and reflection and refraction/5/en

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Next let us derive graphically the law of refraction by using Fermat’s principle. In Figure 21, ST is a plane interface separating two media of velocities and is a normal to this plane. A and B are any two points in the plane normal to the plane ST and containing the normal PC. C is any point on the line ST. D is a neighboring point on the line ST. The time over the path ACB is