Editing Fermat's principle (section)

=== A ray as a signal path (line of sight) ===

If the corridor of paths reinforcing a ray path from ''A'' to ''B'' is substantially obstructed, this will significantly alter the disturbance reaching ''B'' from ''A'' &ndash; unlike a similar-sized obstruction ''outside'' any such corridor, blocking paths that do not reinforce each other. The former obstruction will significantly disrupt the signal reaching ''B'' from ''A'', while the latter will not; thus the ray path marks a ''signal'' path. If the signal is visible light, the former obstruction will significantly affect the appearance of an object at ''A'' as seen by an observer at ''B'', while the latter will not; so the ray path marks a ''line of sight''.

In optical experiments, a line of sight is routinely assumed to be a ray path.<ref>See (e.g.) [[#ToL|Huygens, 1690, tr.&nbsp;Thompson]], pp.{{nnbsp}}47,{{px2}}55,{{px2}}58,{{px2}}60,{{px2}}{{nowrap|82–6}}; [[#Opticks|Newton, 1730]], pp.{{nnbsp}}8,{{px2}}18,{{px2}}137,{{px2}}143,{{px2}}166,{{px2}}173.</ref>