# Fermat’s principle and reflection and refraction

Series | Geophysical References Series |
---|---|

Title | Digital Imaging and Deconvolution: The ABCs of Seismic Exploration and Processing |

Author | Enders A. Robinson and Sven Treitel |

Chapter | 1 |

DOI | http://dx.doi.org/10.1190/1.9781560801610 |

ISBN | 9781560801481 |

Store | SEG Online Store |

According to Fermat’s principle, the propagation of a seismic wave between any two points *A* and *B* will take place in such a way that the first-order infinitesimal variation of the traveltime over the raypath between the two points will be zero with respect to all conceivable neighboring paths. The material between the two points might consist of any number of media with different velocities. The raypaths, therefore, might include any number of reflections and refractions. As a consequence of Fermat’s principle, it follows that the traveltime over a true raypath can be a maximum, a minimum, or stationary with respect to all neighboring paths that start and end at the same two points *A* and *B*. In most practical cases, the traveltime is a minimum. For instance, when the medium is uniform, the raypath is a straight line, which is the shortest distance between the two points, so the time is a minimum. In addition, the time is always a minimum when the interfaces separating the media of differing velocities are plane surfaces. In other cases, it can be a maximum. Fermat’s principle has been used widely to derive useful properties of raypaths. For instance, it can be used to derive the laws of reflection and refraction from plane surfaces.

First let us give a graphic derivation of the law of reflection using Fermat’s principle. In Figure 20, *ST* is the trace (in the plane of the paper) of a reflecting interface that is a plane perpendicular to the plane of the paper. Points *A* and *B* are any two points in the plane of the paper above the plane *ST*. Point is the image of point *A* with respect to the plane *ST*. It is located by drawing normal to *ST* and making *AD* equal to . Draw the straight line , which cuts the line *ST* at point *C*. Let be any point whatever in the plane *ST* that is not coincident with *C*. (Note that is not necessarily on the line *ST*.) Then *ACB* and *AC**B* are two conceivable travel paths from *A* to the plane *ST* to *B*.

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.

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

**(**)

If *CE* is normal to *AD* and *DF* is normal to *CB* and if *CD* is very small, the time over the path *ADB* can be written as

**(**)

so that

**(**)

Now if is such that its first-order variation is zero, we must have

**(**)

and when this condition is satisfied, we can see from the figure that the traveltime will be a minimum and not a maximum. This is Snell’s law of refraction, which already has been derived by using Huygens’ principle.

If point *C* is displaced to either side and at right angles to the line *ST* so that it still remains in the plane *ST*, the traveltime over the displaced path will be greater than that over the undisplaced path. This is true for any arbitrary undisplaced position of point *C* on the line *ST*. It follows that the absolute minimum time occurs when *C* lies on the line *ST*, and therefore the refracted ray *CB* lies in the same plane as the incident ray *AC* and the normal *PC*.

Fermat’s principle states that a raypath between two points separated by several media of different velocities is a path such that the traveltime is stationary to the first order of differentials with respect to all conceivable neighboring paths. Huygens’ principle can be used to derive the law of reflection and Snell’s law of refraction. These same laws also can be derived by using Fermat’s principle, as we have done in this section.

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## Also in this chapter

- Introduction
- Wavefronts and raypaths
- d’Alembert’s solution
- One-dimensional waves
- Sinusoidal waves
- Phase velocity
- Wave pulses
- Geometric seismology
- The speed of light
- Huygens’ principle
- Reflection and refraction
- Ray theory
- Fermat’s principle
- Diffraction
- Analogy
- Appendix A: Exercise