Which phenomenon continues to emit light after the excitation stops?

• A. Fluorescence
• B. Luminescence
• C. Phosphorescence
• D. Color Center

Answer: (C)

Phosphorescence is the phenomenon that continues to emit light after the exciting source is removed. It happens because the excited electrons can become trapped in a metastable state, often a triplet state, and the transition back to the ground state is spin-forbidden. This slow, forbidden transition means light is released over a longer period, creating an afterglow that can last from microseconds to hours depending on the material.

Fluorescence, by contrast, emits while the excitation is present and dies out almost instantly once the light is off, because the electrons return quickly from the singlet excited state to the ground state. Luminescence is a broad umbrella term for light emission due to various excitations, but it doesn’t specify whether the emission persists after excitation stops. Color centers can cause luminescence, but the classic persistence after excitation is the hallmark of phosphorescence.