a
Let the length of segment is " \(\ell^{\prime \prime}\)

Let \(N\) is the no. of fringes in " \(\ell^{\prime \prime}\)

and \(w\) is fringe width.

\(\rightarrow\) We can write

\(N w =\ell\)

\(N \left(\frac{\lambda D }{ d }\right)=\ell\)

\(\frac{N_{1} \lambda_{1} D}{d}=\ell\)

\(\frac{ N _{2} \lambda_{2} D }{ d }=\ell\)

\(N _{1} \lambda_{1}= N _{2} \lambda_{2}\)

\(16 \times 700= N _{2} \times 400\)

\(N _{2}=28\)