The following diagram indicates the energy levels of a certain atom when the system moves from 2E level to E, a photon of wavelength λ is emitted. The wavelength of photon produced during its transition from $\frac{4E}{3}$ level to E is :

3λ

$2 E-E=\frac{h c}{\lambda} \Rightarrow E=\frac{h c}{\lambda}$

$\frac{4 E}{3}-E=\frac{h c}{\lambda '} \Rightarrow \frac{E}{3}=\frac{h c}{\lambda '}$   ∴ $\frac{\lambda '}{\lambda}=3 \Rightarrow \lambda '=3 \lambda$