1. The problem statement, all variables and given/known data I'm having difficulty understanding why this equation makes sense for the following m values. 2. Relevant equations The following is stated in my textbook. For dark fringes, sinθ = (mλ)/a where λ is the wavelength of light in nanometers and a is the width of the single slit. m is ±1, ±2, ±3, ±4, etc.. 3. The attempt at a solution I understand where the equation arises from, however, what I am not understanding is why all values of m are used in this equation and not just the odd ones. Because if I go backwards, here is what happens: The path length difference is (a/2)sinθ, and for destructive interference, the following must hold: path length difference = (m/2)λ, where m is an odd integer because the waves have to be out of phase to destroy each other through superposition. So this is true then: (a/2)sinθ = (m/2)λ but m has to be odd. My book cancelled the twos in the denominator and divided by a on both sides, but made it so all values of m could work. I find this very confusing. Why isn't it only odd values of m?