Question

How is a double slit diffraction pattern different from that of a single slit?

Answer 1

In a single slit diffraction, light spreads out in a line perpendicular to the slit. No particular interesting phenomena are observed.

But in a double slit diffraction, light diffracts when passing though the slits, but the light waves coming out from those slits interfere with each other to producte an interference pattern on the screen. The light is spread out in a line, like in the single slit, but here there is interference, producing regions of constructive (bright fringes) and destructive (dark fringes) interference and a very bright spot at the on the screen, called the central maxima.

So, looking at diffraction only, there is no difference between single slit and double slit because in both cases diffraction happen; but in a double slit there is diffraction as well as interference among the diffracted rays.