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Unformatted text preview: Chapt. 36 Interference 275 where n = c/v ( s ) is the index of refraction which is always greater than 1, except in a vacuum. What does stationary mean here? Just what it means in calculus: the path is constant with respect to first order perturbations. All the light starts in phase from A and flows along the stationary path from A to B and along those continuum of paths that are close to the stationary path will arrive with nearly the same phase. This light will interfere mostly constructively. Light that comes along other paths will arrive with a wide range of phases and will strongly trend to cancel outI think the trend to cancellation is tricky to show in general, but nature has no problem doing it. Note we also have principle of reversibility: a stationary path from A to B is also one from B to A . It is beyond our scope to study methods for finding the stationary path in general. That involves variational calculus. Suffice it to say here that if index of fraction is constant theinvolves variational calculus....
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- Fall '06