What happens to the wavefunction in an allowed transition when an electron transfers from a higher energy level to a lower energy level inside a one-dimensional box?
For simplicity, assume that there are 2 levels in your system (it can be any system you want as long as the particle can transition between the two). When you shine a light on the system, the particle periodically moves between the two states (if the light is monochromatic and at the wavelength of the transition). So after a certain time T, the particle has a 100% probability of being in the upper state, the wavefunction consists only of the upper state. This is called Rabi Oscillation and you can learn more using these resources: