In the experiment we say that light coming from the slits at center part of the screen are in phase. Let at some time amplitude of both wave are at crest, then a maximum bright spot will be formed. But after some time both will have zero amplitude. Then at that time intensity will be lowest. Similarly as the amplitude will be varying with time so the intensity of central part must also varying with time sinusoidal, but this actually does not happen. Why ?

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    $\begingroup$ Can you see an intensity that flutuates at $10^{15}$ Hz. I have difficulty with the $100$ Hz lightbulb flicker. $\endgroup$
    – mike stone
    May 11, 2020 at 12:56
  • $\begingroup$ there are so many questions with very similar title: have you searched for duplicates? $\endgroup$ May 11, 2020 at 12:58
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    $\begingroup$ Does this answer your question? Why does interference pattern remain constant? $\endgroup$
    – Sandejo
    May 11, 2020 at 13:07
  • $\begingroup$ Welcome to PSE. Good question. Unfortunately it was asked many times and it probably will be closed. That has nothing to do with you, so stay on PSE and read and ask more. $\endgroup$ May 18, 2020 at 6:05

1 Answer 1


You see a water wave because the water moves up and down, not because it stays high. The electric field (to pick one) must change from minimum to maximum for there to be a light wave. The bright spots are where there is a large variation in the field, not where the field is large.


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