1. The problem statement, all variables and given/known data A light source shines light of wavelength 490nm onto a pair of slits separated by 0.44m. Calculate the angular location and the location in cm of the second order dark fringes on a screen 1.4m from the slit 2. Relevant equations Xn = (n-1/2)L lambda/d delta x = L lambda/d 3. The attempt at a solution So I understand how to find the angular location, but I don't understand why only one method for finding the location works. The method which works is using the formula containing Xn which gives the answer of 0.23cm. I don't understand why it is not possible given that this is double slit to use delta X = L lambda/d then multiply that answer by 2 to get your answer as it gives a different value. X2=(2-1/2)(1.4)(4.9x10-7)/0.44x10-3 = 0.23cm (the answer in the back of the textbook) Using ΔX = Lλ/d ΔX = (1.4)(490x10-9)/0.44x10-3 = 0.0015 as it is to the second dark fringe, shouldn't 0.0015x2 get me the same answer as my previous Xn calculation?