An atom emits a photon when one of its electrons:
collides with another of its electrons
exchanges quantum states with another of its electrons
undergoes a transition to a quantum state of lower energy
undergoes a transition to a quantum state of higher energy

undergoes a transition to a quantum state of lower energy

Explanation:

When electrons in an atom move to another quantum state, they emit/absorb a photon according to the following:

- If the electron is moving to a higher energy state, it absorbs a photon (because it needs energy to move to a higher energy level, so it must absorb the energy of the photon)

- if the electron is moving to a lower energy state, it emits a photon (because it releases the excess energy)

In particular, the energy of the absorbed/emitted photon is exactly equal to the difference in energy between the two levels of the electron transition:

1/0.14nm = 10^10/1.4 a lot

0.55x745.7j and a calculator