Final answer:
In quantum wells, the parity of an electron changes when it absorbs or emits electromagnetic radiation, due to conservation of angular momentum and the wave-particle duality of electromagnetic radiation.
Step-by-step explanation:
In quantum wells, when an electron absorbs or emits electromagnetic radiation, its parity must change. This is due to the conservation of angular momentum, specifically the change in the electron's orbital angular momentum. When an electron absorbs a photon and transitions to a higher energy level, its parity changes to preserve the total angular momentum of the system. Similarly, when an electron emits a photon and transitions to a lower energy level, its parity also changes. This phenomenon is consistent with the wave-particle duality of electromagnetic radiation and quantum mechanics.