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One of the expressions for the laws of propagation of light is Fermat's principle, which states that light always travels along the shortest path. Consider the following case. Light leaves a source A (Fig.), is reflected from a mirror, and passes through point B; show that the path ACB determined by the law of reflection is the shortest of all other possible reflected paths for the ray.

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For the proof it is necessary to construct the image of point A in the mirror and consider the ratio of the lengths of paths A1CB and A1DB (Fig.).

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