Final answer:
Delayed emission of light occurs in phenomena like phosphorescence, where light energy absorbed by a substance is released over a longer period due to metastable states, and in thermoluminescence, where light is released from energy trapped in materials like ceramics.
Step-by-step explanation:
The term delayed emission of light refers to the phenomenon where light is released from a substance some time after it has absorbed energy. This is typically observed in phosphorescence, a type of photoluminescence related to fluorescence. However, unlike fluorescence, where the emission of light is almost immediate, phosphorescence involves energy being released over a longer period due to metastable states. These metastable states are where the electrons have been excited to a higher level and there is a delay in returning to their ground state because the transition is quantum mechanically forbidden, or less favorable.
Thermoluminescence is another form of delayed light emission. This occurs in certain ceramics, where the atomic excitation energy can be 'trapped' after the ceramic has been fired and cooled. Later, by heating the ceramic, it can be made to release this stored energy as light. The amount of light emitted can be used to date archaeological finds, such as pottery, because the light emission decreases over time. Hence, the less light emitted, the older the artifact is assumed to be.
In short, the delayed emission of phosphorescent light can be due to the decay of metastable states, and thermoluminescence is the delayed emission of light due to trapped excitation energy in certain materials.