Britannica Sources used in this answer

How do electronic changes in atoms produce visible light and ultraviolet radiation?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Diagram of a tree leaf illustrating photosynthesis, with arrows showing sunlight, water, and carbon dioxide entering the leaf and sugars and oxygen being produced. A person is shown beside the tree to demonstrate the exchange of oxygen and carbon dioxide between plants and animals. — electromagnetic radiation Diagram of photosynthesis showing how water, light, and carbon dioxide are absorbed by a plant to produce oxygen, sugars, and more carbon dioxide.

Visible light and ultraviolet radiation are produced through the movement of electrons between discrete energy levels within an atom or molecule.12

According to quantum mechanics, electrons occupy specific, allowed energy states. When an atom is excited—often by the impact of particles or the absorption of energy—an electron is promoted from a lower-energy ground state to a higher-energy excited state. As the electron spontaneously returns to a lower energy level, it releases the excess energy as a photon of [electromagnetic radiation].1234

The energy of the emitted photon ( $E$ ) is equal to the difference between the two energy levels, which determines the frequency and wavelength of the radiation according to the relationship $E = h ν$ (where $h$ is Planck’s constant).23
* Visible light is emitted when electronic transitions correspond to energy differences typically ranging from 1.8 to 3.1 electron volts.3
* Ultraviolet radiation is produced by transitions involving larger energy differences, such as those observed in mercury atoms, where electrons dropping to lower states emit high-energy photons at wavelengths like 185 or 254 nanometres.3