# Translations:Fermat’s principle and reflection and refraction/26/en

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 **Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle A^'}**
is the image of point *A* with respect to the plane *ST*. It is located by drawing **Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle AA^'}**
normal to *ST* and making *AD* equal to **Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle DA^'}**
. Draw the straight line **Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle A^'CB}**
, which cuts the line *ST* at point *C*. Let **Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle C^'}**
be any point whatever in the plane *ST* that is not coincident with *C*. (Note that **Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle C^'}**
is not necessarily on the line *ST*.) Then *ACB* and *AC***Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle C^'}**
*B* are two conceivable travel paths from *A* to the plane *ST* to *B*.