Why is there a single slit in the Young double-slit experiment?

Question

I am studying waves these days and my teacher just introduced Young double-slit experiment, which has always been brilliant (see picture below).

However, I have a question: why is there a single slit in the experiment?

My teacher says that the single slit is used to make sure the light at both of the double-slit are coherent (i.e. they have same frequency).

But what I think is that, since the light source is monochromatic (i.e. the light has only one frequency), theoretically, any two light rays from this source must be coherent. So it's very unnecessary to use the single slit. I believe there must be some other reason for this single slit.

Can anyone help me out?

Answer 1

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 and varying with time in different ways.

The single source is then split into two with the two slits. That creates two sources of the same frequency and constant phase difference - i.e. coherent.

If the light from the original source landed directly onto the two slits, the two slits are not guaranteed to be coherent sources.