Final answer:
Emission of light from an atom occurs when an electron loses energy. This process is governed by the law of conservation of energy.
Step-by-step explanation:
Emission of light from an atom occurs when an electron loses energy. When an electron transitions from an excited state to a less excited state or the ground state, the difference in energy is emitted as a photon. This process is governed by the law of conservation of energy, which states that the same amount of energy absorbed by an electron to excite it will be released as a photon when the electron returns to its initial state.