\(\frac{I_{1}}{I_{2}}=\frac{16}{9}=\frac{a_{1}^{2}}{a_{2}^{2}}\)
\(\Rightarrow a_{1}=4 ; a_{2}=3\)
Therefore the ratio of intensities of bright and dark parts
\(\frac{{{I_{{\text{Bright }}}}}}{{{I_{{\text{Dark }}}}}} = \frac{{{{\left( {{a_1} + {a_2}} \right)}^2}}}{{{{\left( {{a_1} - {a_2}} \right)}^2}}}\) \( = \frac{{{{(4 + 3)}^2}}}{{{{(4 - 3)}^2}}} = \frac{{49}}{1}\)