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Fluorescence and phosphorescence are both forms of luminescence, where a substance emits light after absorbing energy. The key difference lies in the energy states involved:
Fluorescence: The absorbed energy excites an electron to a higher energy level, and it quickly returns to its ground state, emitting a photon of light. This process is rapid, typically lasting less than 10^-8 seconds.
Phosphorescence: After absorbing energy, the electron transitions to a higher energy level and then falls into a metastable excited state called a triplet state. This state is longer-lived than the singlet state in fluorescence, and the electron can stay in this state for seconds or even minutes before returning to the ground state, emitting a photon of light. This delayed emission is what gives phosphorescence its characteristic "glow-in-the-dark" effect.