So in Young's Double slit experiment the source of light that passes through the double slit must be monochromatic and a coherent source (from my undertanding, this is to get a uniform pattern projected on a screen/wall/whatever, correct me if I am wrong). Young without having access to lasers used a light filter to filter light for it be of one wavelength and then exposed it to a single slit for a coherent source. From what I understand, a coherent source of light is a wave which has a constant phase/path difference, meaning that at any point on the wave there is the same phase/path difference as any other point on the same wave. However, I can't figure out why diffracting a monochromatic source of unpolarised light would cause the resultant wave to be coherent, as I am unable to see where phase/path difference are relevant.
Any help is appreciated. Thanks!