How do ultraviolet rays make some phosphors emit visible light?

Ultraviolet (UV) rays induce visible light emission in phosphors through a process of electronic excitation. In inorganic phosphors, the electronic energy levels form bands, with electrons typically residing in the valence band in their ground state.1

When a phosphor is bombarded by UV photons, the energy is absorbed, causing electrons to be promoted to the conduction band. Within the crystal lattice, activator ions or centers create additional energy levels that bridge the energy gap between the valence and conduction bands. As an excited electron is transferred to these intermediate levels and subsequently returns to its ground state, it releases the stored energy in the form of visible light.1

This mechanism is fundamental to technologies like fluorescent lamps, where UV radiation—such as the 254-nanometre emission from mercury vapor—is used to excite phosphor coatings, which then emit visible light. The specific color of the emitted light depends on the chemical nature of the host crystal and the specific activator ions present.12