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Two very narrow slits are spaced apart and are placed 35.0 cm from a screen. What is the distance between the first and second dark lines of the interference pattern when the slits are illuminated with coherent light with λ= 550nm

I'm not asking for an answer for this because I understand the method. I am confused by the theory behind it. When I consulted the solutions manual for this question, they say the first dark line occurs at m = 0 (where m is the order of the image), whereas I thought it occurred at m = 1. Why is this so?

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  • $\begingroup$ Can you define $m$ please? There is a point of constructive interference in line with the direction of the incident source, yes. $\endgroup$ – Floris Apr 24 '16 at 17:50
  • $\begingroup$ And of course one has to define "first" dark line - I presume it is counting up from the center out... $\endgroup$ – Floris Apr 24 '16 at 17:51
  • $\begingroup$ Different people can mean different things by $m$. Some people start counting at 0, others at 1 ... in any event, we need to know what it means to you. $\endgroup$ – garyp Apr 24 '16 at 19:56
  • $\begingroup$ Sorry about that. In this instance "m" is referring to the order of the image. I'll amend my question. $\endgroup$ – BigBoss Apr 25 '16 at 11:17
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It's not m=0. The sin of d ( or the angle that you will find the dark spots) is equal to ( m+1/2) times the wavelength. m=0,1,2,3.........

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  • $\begingroup$ This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review $\endgroup$ – Jon Custer Apr 24 '16 at 21:14
  • $\begingroup$ Sure it does. My answer gave the correct formula for the dark spots. The OP thought it occurred at in m=1. There is more to the formula than that. How else would you have answered it? $\endgroup$ – Bill Alsept Apr 24 '16 at 22:51
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Apologies everyone, I misunderstood this question greatly. It's talking about the first dark fringe (or the zeroth order dark fringe) which occurs in between 0 and 1. I understand now.

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