If we send a laser beam through a pair of slits, what should we see?
A series of bright spots, equally spaced and equally bright.
Well, here's a picture of the same two-slit experiment you performed last week.
Q: Does the picture agree with the theory?
No! It does not. The spots aren't all equally bright; instead, they appear to go through a series of oscillations, bright to faint to bright again, which eventually fade away.
Q: Does that large-scale pattern remind you of anything?
Yes, of course: the single-slit diffraction pattern.
See how well they agree?
Each of the two slits in the double-slit experiment produces a large-scale diffraction pattern. That pattern modifies the simple two-slit interference pattern we expect from theory, so that the actual two-slit experiment yields a combination of the patterns.
The wavelength of this laser was 660 nm. The width of the single slit was 0.02 mm. The width of each of the double slits was 0.02 mm. Q: What was the separation between the slits of the two-slit experiment?