Why is a light wave diffracted when it passes a narrow slit?

Answer 1

Same reason all waves are diffracted.... not sure what that reason IS, to be entirely honest. We were doing something in physics about wavelets, which is a theory that says that every point on the front of a wave gives off little wavelets and where these overlap the next wave front is formed.... hard to explain without a diagram.

The reason light requires a narrow slip in order to diffract noticeably is that it has a very small wavelength, and waves diffract most when the gap they are passing through is the same as their wavelength.

Answer 2

Turtle's answer covers the ground well and I would like to add that Fresnel's diffraction and Fraunhofer's diffraction are the two phenomena. Fresnel's diffraction takes place at an edge and you can think of Fraunhofer's diffraction through a slit as two edges near each other so that the light from the edges can interfere. The points on the edge give off waves and the wavefronts from different points interfere giving rise to fringes.