Do slits cause light to become coherent? In the young double slit experiment, you have a single slit followed by two slits. I believe it is the single slit which cause the light to diffract and become coherent and the double slit is to create two different sources. However, in my textbook, it says that it is difficult to to get a coherent source of microwaves and they use double slits to diffract the microwaves and it says that is what makes the microwaves coherent. If that is the case, why does the double slit experiment bother to use a single slit as the double slit would cause the light to be coherent or why isn't a single slit then a double slit used in the microwave experiment?
 A: The single slit is used to create light from a single source.  Even what we regard as a single source of light e.g. a bulb can have different parts to it, e.g. different parts of the filament, emitting light at different frequencies and amplitude varying in time in different ways.
The single source is then split into two with the two slits that create two sources of the same frequency and constant phase difference i.e. coherent.
If the original light source landed directly onto the two slits, the two slits are not guaranteed to be coherent sources.
If your textbook describes a similar experiment using microwaves and only the double slits, then presumably the source of the microwaves is 'pure' enough to not need the single slit.
A: In a double slit experiment, the wave packet associated with each photon is split into two parts which interfere with each other at points within the interference pattern. A line source is used so that the two parts have a fixed phase difference when they arrive at the two slits.  Different wave packets do not have to have a fixed phase relationship with each other, but for a double slit they should all have the same frequency. In an interferometer, if the difference in the two paths is to large, one part of the packet will be gone before the other arrives, and the interference pattern disappears (unless your source is a laser beam).  When working with a multiple slit grating (which gives sharply defined maxima with wide angular separations), an incandescent bulb with a line filament (which produces light with no coherence) works fine as a source ( and produces a nice spectrum).
