The spacing between the dark regions is constant. The width of the central maximum is approximately twice the width of the subsidiary maxima, which are also nearly evenly spaced. All other things being equal, the overall width of a diffraction pattern is inversely proportional to the slit width. In Young's experiment the two point sources of light were in phase. Dark bands are regions of destructive interference.
Bright bands are regions of constructive interference.
Each slit produces a single slit diffraction pattern. These two patterns interfere with each other, producing evenly spaced bright fringes superimposed on the background single slit pattern. All other things being equal, the spacing of bright fringes is inversely proportional to the slit spacing, d . The path difference is the difference in the distance light travels from each slit to the screen. It can be expressed in wavelengths.