The top right of this physlet shows the double source pattern of two coherent sources, separated by a distance d. Then there is a single slit diffraction pattern, with a slit width of a. The bottom graph shows the effect of having two slits of width (a), separated by a distance (d). Notice that the single slit "envelope" is multiplied by the double slit pattern to form the final 'real' double slit pattern.
To find the overall intensity of the double-slit pattern, simply multiply the double-source intensity by the single-slit intensity :
I = Imax cos2 (pd sin(q)/l) [sin (pa sin(q)/ l) / (pa sin(q)/ l) ]2
When we use the double-source equation to find locations of bright spots, we find that there are some places where we expect to see bright spots, but we see no light. This is known as a missing order, and it happens because at that location there's a zero in the single slit pattern. [Note, if the zero in the single slit pattern, and a zero in the double slit pattern, it is not called a missing order .. for, there is no order to be missing! Also, if there is a local peak in the single slit pattern, and a zero in the double source pattern, there will still be a zero (remember, we multiply the functions!) - this also is not a missing order.]
Scripts modified by Scott Schneider at LTU (from scripts authored by Andrew Duffy at Boston University).