We know that the fringe width (β) in the interference pattern is given by, \(\beta=\frac{\lambda D}d\), where D is the distance between slits and the screen, d is the separation between the slits and λ be the wavelength of light.
From the above expression, it is clear that, \(\beta\propto\frac1d\)
If the coherent sources are close to each other i.e., for small d, the fringe width will be large, and hence good interference patterns can be obtained.
