1. The problem statement, all variables and given/known data How many nodal lines are created by two point sources emitting at the same wavelength that are separated by a distance equal to 2.5 lamda? 2. Relevant equations My textbook says, "If two coherent sources located a distance d apart emit identical waves of wavelength lamda, then the number of nodal lines on either side of a straight line running through the centers of the sources is the greatest integer smaller than or equal to 2(d/lamda)." 3. The attempt at a solution Using the formula from the textbook I get 5 nodal lines. But this answer is incorrect. I've also tried 3 based on the fact that nodal lines occur in places where the path difference from the sources are some 0.5 lamda, 1.5 lamda, 2.5 lamda,... etc. I'm thinking based on the quote from the text book that I need to double my answer of five since the answer is just for one side of a line running through the center of the sources. I really don't want to lose anymore credit however. Any hints?