When an excited atom returns to its normal state, it emits a single photon rather than two photons or a particle due to the conservation of energy and momentum in quantum systems.
In quantum mechanics, energy levels of atoms are quantized, meaning they can only occupy certain discrete energy states. When an atom is excited to a higher energy state, it transitions to an unstable state, which is not a permanent or stationary state. The atom then undergoes a spontaneous decay process, transitioning back to its lower energy state.
During this transition, the atom releases the excess energy it gained when it was excited. This energy is carried away by a single photon. The photon's energy is precisely determined by the energy difference between the initial and final states of the atom.
The conservation of energy is crucial in this process. If two photons were emitted instead of one, their combined energy would have to match the energy difference between the energy states involved. However, this scenario violates the conservation of energy because the energy of each individual photon would have to be half of the required energy. Since energy is quantized, it cannot be split in this manner.
Similarly, the conservation of momentum also plays a role. A single photon carries a definite amount of momentum, and when it is emitted from the atom, momentum must be conserved. Emitting two photons or a particle with mass would result in a different momentum distribution and violate the principle of momentum conservation.
Therefore, to conserve both energy and momentum in accordance with the laws of quantum mechanics, an excited atom emits a single photon as it transitions back to its normal state.