What is UV light: Ultraviolet (UV) is electromagnetic radiation with wavelength from 10nm to 400nm, shorter than that of visible light but longer than X-rays.
UV radiation is present in sunlight, and constitutes about 10% of the total electromagnetic radiation output from the Sun.
UV light can cause chemical reactions and causes many substances to glow or fluoresce.

In gemmology, two energy levels of UV light are used:
Long wave ultraviolet (LWUV) 365nm | Short wave ultraviolet (SWUV) 254nm

Visibility: Ultraviolet rays are invisible to most humans. The lens of the human eye blocks most radiation in the wavelength range of 300-400nm; shorter wavelengths are blocked by the cornea. Humans also lack color receptor adaptations for ultraviolet rays.

UV in Gems world: In gemstone world, different minerals may exhibit different fluorescence or phosphorescence reaction under UV. Other than the beauty that it gives out, it can be used as a back-up to identify the gems.

Luminescence: it describes the emission of 'cold', non-incandescent visible light by a substance. Both fluorescence and phosphorescence are kind of luminescence.

Fluorescence: It is the emission of visible light by a substance while it is being illuminated by radiation of higher energy. And the effect will exhaust once the outside influence is removed.

Phosphorescence: is a type of luminescence, phosphorescent materials continue to emit radiation after the removal of the source of irradiation.