A wavefront undergoes a phase change as it travels along the fiber. If the wavefront transverses the fiber twice and is reflected twice and the total phase change is equal to 1/2 p, will the wavefront disappear? If yes, why?

A wavefront undergoes a phase change as it travels along  the fiber. If the wavefront transverses the fiber twice and is  reflected twice and the total phase change is equal to 1/2 p, will  the wavefront disappear? If yes, why?
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Answer :

Yes, the wavefront will disappear because the total amount of  phase collected must be an integer multiple of 2 p. (If the propagating  wavefronts are out of phase, they will disappear. The wavefronts that  are in phase interfere with the wavefronts out of phase. This type of  interference is called destructive interference.)
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