Energy levels $A, B$ and $C$ of a certain atom corresponding to increasing values of energy i.e. $E_A \lt E_B \lt E_C$. If $\lambda_1$, $\lambda_2 \& \lambda_3$ are the wavelengths of a photon corresponding to the transitions shown, then

• A

  $\lambda_3=\lambda_1+\lambda_2$
• B

  $\lambda_3=\frac{\left(\lambda_1+\lambda_2\right)}{\lambda_1 \lambda_2}$
• C

  $\lambda_3^2=\lambda_1^2+\lambda_2^2$
• Correct answer
  D

  $\lambda_3=\frac{\lambda_1 \lambda_2}{\left(\lambda_1+\lambda_2\right)}$

Hence, the correct answer is option: 'D'.