Intensity for a Double Slit

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 at some places where we expect to see bright spots we see no light. This is known as a missing order, and it happens because at that location there's a zero in the diffraction pattern.