Diffraction at a double slit and multiple slits

The experiment P5.3.1.2 explores diffraction at a double slit. The constructive interference of secondary waves from the first slit with secondary waves from the second slit produces intensity maxima; at a given distance d between slit midpoints, the angles ϑ_n of these maxima are specified by

The intensities of the various maxima are not constant, as the effect of diffraction at a single slit is superimposed on the diffraction at a double slit. In the case of diffraction at more than two slits with equal spacings d, the positions of the interference maxima remain the same. Between any two maxima, we can also detect N-2 secondary maxima; their intensities decrease for a fixed slit width b and increasing number of slits N.