My Book says that

The following conditions should be satisfied for a sustained interference

  1. The two coherent sources placed in front of two slits separated by some distance should be close to each other so as to produce a sustained interference on the screen kept at some distance from the slits

  2. Amplitude of incoming waves should be equal

  3. The two sources should be strong with least background

I have got the following justification for the upper given point

  • For the first point maybe if the current sources are not close to each other than the phase difference changes continuously and won't give interferance

  • according to me the second point is wrong because even if the amplitude of waves are not equal still I have seen interference pattern forming

  • I didn't understand the third point

Please help me

  • $\begingroup$ Which book? ___ $\endgroup$ Commented Jan 5, 2019 at 12:38
  • $\begingroup$ Which book? Can you give an author and full title, or a link? $\endgroup$ Commented Jan 5, 2019 at 12:51

1 Answer 1


I think that they give "practical conditions" to observe interferences : If the two slit are too far apart, the fringes will be too close to be easily observed (the interfringe is lambda*D/a and a is too great). Moreover there will be a problem with the spatial coherence of the source witch illuminate the slits. If the amplitude are too differents, the contrast of the fringes will be low. If there is highly illuminated background, you will not see the fringe (low contrast !) Hope you can understand my poor enblish !

  • $\begingroup$ Hey in the first point you have talked about two slits where is in my question I have talked about distance between two coherent sources please correct that thanks for the answer $\endgroup$
    – David Wax
    Commented Jan 5, 2019 at 13:24
  • $\begingroup$ @DavidWax You are talking about Young's Double Slit Experiment right? It required only one coherent source not two. $\endgroup$ Commented Jan 5, 2019 at 13:49
  • $\begingroup$ I thought that the two slits were the two "coherent sources" ? I dont understand why they would speak of two slits illuminated by two coherent sources ? $\endgroup$ Commented Jan 5, 2019 at 14:19

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.