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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?

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  • $\begingroup$ Are you asking why wave always travel in one direction? or Why there is no wave travel in opposite direction? $\endgroup$
    – Shing
    Commented Nov 21, 2015 at 10:16
  • $\begingroup$ Did my question help you? $\endgroup$
    – user36790
    Commented Nov 21, 2015 at 10:52
  • $\begingroup$ @user36790 : No, sorry. answer given refers to some 1d or 2d. my question is different. $\endgroup$
    – Anubhav Goel
    Commented Nov 21, 2015 at 11:01
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    $\begingroup$ 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. $\endgroup$
    – Wapiti
    Commented Nov 21, 2015 at 17:21
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    $\begingroup$ @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. $\endgroup$
    – Wapiti
    Commented Nov 22, 2015 at 20:07

1 Answer 1

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Why doesn't a backward wavefront exist?

It exists. See the attached picture.

enter image description here Source

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  • $\begingroup$ Sorry! I wanted to ask about a backward wave. $\endgroup$
    – Anubhav Goel
    Commented Nov 21, 2015 at 10:51
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    $\begingroup$ This is silly. If the backwards wave existed then you would hear echoes every time somebody talked, and you would see a lightbulb continue to glow after it was turned off (among other things). You can draw a picture, but it proves nothing. $\endgroup$
    – Wapiti
    Commented Nov 21, 2015 at 17:23
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    $\begingroup$ @Wapiti: paradaillan is correct actually; check the pic. But it doesn't answer the question unfortunately due to possible misinterpretation of the question. $\endgroup$
    – user36790
    Commented Nov 21, 2015 at 18:03
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    $\begingroup$ Yeah, I saw the picture. Like I said it proves nothing. Would YOU take the picture as proof the backwards wave exists? $\endgroup$
    – Wapiti
    Commented Nov 21, 2015 at 18:06
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    $\begingroup$ The figure shows that there is a "backward" solution. Which is true as far as it goes, but any given, physical wave only exhibits one solution at a time, which is what the question asks about. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Nov 22, 2015 at 1:37

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