In order to find intensity distribution, we have to calculate the resultant amplitude of all waves reaching the point P . These wavelets are now originating from N slits and the calculation of the amplitude, as we did in case of single slit and double slit, is complicated. However, we can use the complex amplitude method. Here we note that the phase difference between the waves originating from similar position of two consecutive slits is ( b is separation between the two slits). The amplitude part from each wave is Thus if we have two slits