Explanation: Hints: \(\frac{1}{\lambda} = R_H \left( \frac{1}{n_1^2} - \frac{1}{n_2^2} \right)\) Solve: \(n_1 = 3, n_2 = 4\) \[ \frac{1}{\lambda} = R_H \left( \frac{1}{3^2} - \frac{1}{4^2} \right) = 1.09678 \times 10^{-2} \left( \frac{1}{9} - \frac{1}{16} \right) \] \(\lambda = 1.875 \, \mu m\) Ans. (B) ব্যাখ্যা: তরঙ্গ সংখ্যা \(\overline{v} = \frac{1}{\lambda} = R_H \left( \frac{1}{n_1^2} - \frac{1}{n_2^2} \right)\)। তরঙ্গ দৈর্ঘ্য নির্ধারণ করে।