Atoms emit a photon when an electron falls from a high-energy state to a low-energy state. The conditions under which this process occurs happen in two ways. According to the Cornell Center for Materials Research, electrons either absorb the energy from a photon and jump to a higher energy level or a photon collides with an electron that is already in an excited state.
In both cases, as the electron falls from a high-energy state back down to ground level, a photon is emitted. The light or photon emitted as the electron falls from the high-energy state to the lower state represents the energy difference between the two states. The photon’s wavelength also represents the distance between the two energy states.
Electrons become excited and quickly jump from ground level to a higher energy level by absorbing the energy from a photon. Since electrons seek stability, they fall back down to the stable ground level, thus emitting a photon. This process of electron excitation is known as absorption.
Spontaneous emission, the second method of electron excitation, occurs when a photon collides with an electron that is already residing in a high-energy state. Once again, as the electron falls from the higher energy level to the ground level, a photon of light is emitted.