Final answer:
Without additional information about the energy levels and transitions of the electron, we cannot determine the exact wavelength of the photon it would emit. It could be the same or different than the absorbed photon, depending on the electron transitions within the atom.
Step-by-step explanation:
The emission wavelength of a photon by an electron depends on the specific energy levels involved in the electron transitions within the atom. When an electron absorbs a photon, it moves to a higher energy state. Eventually, the electron will return to a lower energy state, emitting a photon in the process. The wavelength of the emitted photon could be the same as the absorbed one if the electron transitions back to its original energy state, emitting a photon of 400 nm. However, without additional information about the energy states and transitions, it is impossible to determine the exact wavelength of the emitted photon. It's worth noting that in a multi-step transition, the emitted photons can have different wavelengths that are longer than the absorbed photon due to energy being lost in non-radiative ways between steps.