CBSE Class 12 · 2022 · Wave OpticsShort Answer

(i) $\text{Screen moved away}$: $\beta =\frac{\lambda D}{d}\propto D$, so moving the screen away increases $D$ and the fringe width increases.
(ii) $\text{Source slit moved away}$ from the double slit: this increases its distance $S$, improving the condition $\frac{s}{S}<\frac{\lambda }{d}$, so the fringes become sharper and better defined (greater contrast).
(iii) $\text{Constant extra phase of }\pi$: a fixed phase difference of $\pi$ shifts the whole pattern by half a fringe. The point that was the central bright fringe now has a net phase difference of $\pi$ and becomes dark; bright and dark fringes simply interchange positions while the fringe width stays $\beta =\frac{\lambda D}{d}$.