Location of \(1^{\text {st }}\) minima

\({y_1} = \frac{{D\lambda }}{a} = 0.2469\,D\lambda \)

Location of \(2^{\text {nd }}\) minima

\({{\text{y}}_2} = \frac{{2{\text{D}}\lambda }}{{\text{a}}} = 0.4938\,{\text{D}}\lambda \)

Now for interference

Path for interference

Path difference at \(P\)

\(\frac{d y}{D}=4.8 \lambda\)

Path difference at \(P\)

\(\frac{d y}{D}=9.6 \lambda\)

So orders of maxima in between \(\mathrm{P}\) and \(\mathrm{Q}\) is

\(5,\,6,\,7,\,8,\,9\)

So \(5\) bright fringes all present between \(\mathrm{P}\) and \(\mathrm{Q}\)