Why do some objects glow under ultraviolet light?

Materials that glow under ultraviolet light, also called black light are called to be fluorescent. In normal light emission, photons are released from electrons at the same frequency as they were absorbed. In fluorescence materials this is not the case as they emit light at different wavelengths than absorbed. In normal color emission, electrons absorb energy from photons that is required to move up energy levels. As the electrons fall to their ground state they release the energy in the form of photons at the exact same frequency. Ultraviolet light cannot be seen by human eyes as the frequency is outside the visual light spectrum. When fluorescence materials are exposed to this black light they usually reemit photons with a lower frequency that is within the visual light spectrum. The difference in energy is emitted as heat.

Excitation: S₀ + hv_ex --> S₁

Emission: S₁ --> S₀ + hv_em + heat

(Where S₀ is the ground state and S₁ the first energy level)

These materials usually have rigid structures and delocalized electrons. Examples of materials are called white paper, vitamins, body fluids chlorophyll.