Answer
$$
\approx 4.8 \mathrm{mm}
$$
Work Step by Step
$\text {By using } h c=1240 \mathrm{eV} \cdot \mathrm{nm},$ we get
$$
\lambda=\frac{h c}{\Delta E}=\frac{1240 \mathrm{eV} \cdot \mathrm{nm}}{2.56 \times 10^{-4} \mathrm{eV}}=4.84 \times 10^{6} \mathrm{nm} \approx 4.8 \mathrm{mm}
$$
