For the 3rd dark fringe's path position, the slit should be thought to be divided into how many parts? ___
Hey, you must have recognised the pattern by now. For the 3rd minima (dark fringe), divide the slit into 6 parts, such that each part has a path difference of λ/2. So, you will get asinθ6=λ2 or asinθ=3λ.