The path in empty space

The simplest application of the law of least time occurs in empty space, where light follows a straight line between any two points. Of all the rays which set out from a source, the only one to reach a particular point will be the ray which originally sets out in the right direction, i.e. straight towards the point. That particular ray will have taken the quickest path and, in this case, the quickest path is also the shortest path. In empty space a straight line is the quickest route.

This applies anywhere where the speed of light is constant and there is an unobstructed path between the points.

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