# Why doesn't a backward wave exist? [duplicate]

Huygens principle says every point of wavefront emit wavelet in all directions. Then why does a back ward wave not exist? Can any expert tell real answer? On different sites I get different and contradictory answer.

I am asking question from my class 12 book. It says " Huygens argued that the amplitude of the secondary wavelets is maximum in forward direction and zero in the backward direction; by making this adhoc assumption, Huygens could explain the absence of backwave. However, this adhoc assumption is not satisfactory and the absence of the backwave is really justified from more rigorous wave theory."

What justification is given by wave theory?

answer explained by Marty green on Is Wikipedia wrong about Huygens-Fresnel Principle?

is different from answer given by Acuriousmind on

How does Huygens Principle incorporate the unidirectional property of a traveling wave?

Which is correct?

## marked as duplicate by user36790, Void, John Rennie, Kyle Kanos, ACuriousMind♦Nov 21 '15 at 13:37

• Are you asking why wave always travel in one direction? or Why there is no wave travel in opposite direction? – Shing Nov 21 '15 at 10:16
• Did my question help you? – user36790 Nov 21 '15 at 10:52
• @user36790 : No, sorry. answer given refers to some 1d or 2d. my question is different. – Anubhav Goel Nov 21 '15 at 11:01
• The answer given in the question the OP's question supposedly duplicates does not answer the question at all. It is completely wrong to say that 'this question already has an answer'. The answer given was never accepted, and for good reason. The OP is right to not accept your shutting down the question. If you look in the comments of the original question you linked, you will see that the question was not answered. The reverberation that exists in even dimensions is distinct from backwards waves. This questions actually has an answer, and it has not been given yet. – Wapiti Nov 21 '15 at 17:21
• @Anubhav Goel: Concerning your last additions and links asking which answer is correct -- neither are correct. I have put the correct answer on both pages. – Wapiti Nov 22 '15 at 20:07