Distance of \(1^{\text {st }}\) maxima from central fringe \(=\frac{\lambda D}{d} \quad[D=x]\)
Distance of 1 st minima \(=\frac{\lambda D}{2 d}\)
\(D\) at time \(t=D_t=x+V t\)
\(\frac{\lambda x}{d}=\frac{\lambda(x+V t)}{2 d}\)
\(x=V t\)
or \(\quad t=\frac{x}{V}\)