# Interference Pattern and dark lines

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?

• Can you define $m$ please? There is a point of constructive interference in line with the direction of the incident source, yes. – Floris Apr 24 '16 at 17:50
• And of course one has to define "first" dark line - I presume it is counting up from the center out... – Floris Apr 24 '16 at 17:51
• 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. – garyp Apr 24 '16 at 19:56
• Sorry about that. In this instance "m" is referring to the order of the image. I'll amend my question. – BigBoss Apr 25 '16 at 11:17