As per given figure \( A \), \( B \) and \( C \) are the first, second and third excited energy levels of hydrogen atom respectively. If the ratio of the two wavelengths \( \left(\text{i.e. } \dfrac{\lambda_{1}}{\lambda_{2}}\right) \) is \( \dfrac{7}{4n} \), then the value of \( n \) will be _______.
