Double-slit: what causes the altering problem

[genu]

Homework Statement

It should be obvious that the simple interference of two waves is not the only that you have seen with the double slit. What other phenomenon is altering your pattern?

Homework Equations

The Attempt at a Solution

I'm assuming that the intensity of the light of the laser, but that would not alter the pattern, it would just be less of it. Also the slit separation would contract/expand the pattern, but it would essentially be the same.

 

[Doc Al]

What if there were only one slit?