What do fluorescent substances do when exposed to UV light?

Fluorescent substances have the ability to absorb ultraviolet (UV) light and then re-emit that energy as visible light. This phenomenon occurs because when these substances absorb the higher energy UV photons, they become excited and then release energy as they return to their ground state. The energy emitted during this process is of lower energy than the absorbed UV light, which is why it is emitted as visible light.

This property is utilized in various applications, such as microscopy, where fluorescent dyes are used to label cells or structures, making them easily observable under UV light. The visible light emitted can be observed through specific filters in the microscope, allowing researchers to visualize and study various biological samples with greater clarity.

The other options do not accurately describe the process of fluorescence. For instance, the emission of UV light or reflecting it does not align with the behavior of fluorescent substances, and enhancing contrast pertains to techniques that improve visibility but do not directly describe the emission process itself.