What is phosphorescence?

Phosphorescence refers to the phenomenon where certain materials absorb photons (usually from ultraviolet light) and then re-emit that light over a period of time, even after the excitation source has been removed. This process involves the absorption of energy, which excites electrons to a higher energy state. Unlike fluorescence, where the emitted light ceases almost immediately after the external light source is turned off, phosphorescence can continue for a noticeable duration once the source of excitation is no longer present.

In this case, the correct description is that light is emitted both during and after exposure to ultraviolet light, which captures the essence of how phosphorescent materials behave. This long-lasting emission is often observed in glow-in-the-dark materials, where the glow persists well beyond the time of UV exposure. The other options do not accurately reflect this characteristic of phosphorescent materials, as they either limit the emission of light to the presence of UV exposure or relate to different phenomena entirely.