Shift of fringes is given by \(\frac{(\mu-1) t D}{d}\)

This is equal to position of \(10^{\text {th }}\) bright fringe \(\frac{10 \lambda D}{d}\)

\(\frac{0.6 \times 1 \times 10^{-5} D}{d}=\frac{10 \lambda D}{d}\)

\(\lambda=0.6 \times 10^{-6}\)

\(\therefore \lambda=6 \times 10^{-7}\)

\(\text { or } \quad \lambda=6000 \,\mathring A\)