Fermat's variational principle states that the signal propagates from point S to R along a curve which renders Fermat's functional \documentclass[12pt]{minimal}
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$$\mathcal{I}$$
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$$\mathcal{I}$$
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$$\mathcal{I}$$
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$$\smallint _S^R \mathcal{L}$$
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$$\dot x$$
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$$\dot x$$
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$$\dot x$$
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$$\dot x$$
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$$\dot x$$
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$$\dot x$$
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$$\dot x$$
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$$\dot x$$
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$$\dot x$$
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$$\dot x$$
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$$\dot x$$
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$$\dot x$$
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$$\dot x$$
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$$ - \frac{1}{2}$$
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$$ - \frac{1}{2}$$
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