Final answer:
Fluorescence is observed when comparing neutral, acidic, and basic samples of 4-methylumbelliferone under a black light, as it involves instant re-emission of absorbed ultraviolet light in the visible spectrum.
Step-by-step explanation:
When comparing neutral, acidic, and basic samples of 4-methylumbelliferone under a black light, the phenomenon being observed is fluorescence. Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation. Under a black light, materials that exhibit fluorescence, like 4-methylumbelliferone, absorb the ultraviolet light and then re-emit it almost instantaneously in the visible spectrum. This is different from phosphorescence, where the emitted light is delayed, and chemiluminescence, which occurs through a chemical reaction without the need for light absorption.