Black light (or ultraviolet or UV light) is a part of the electromagnetic spectrum which is divided into ranges from cosmic rays to radio waves. Visible light is a small portion of this spectrum and has wavelengths between about 380 and 700 nanometers. UV light wavelengths span (1 - 340 nanometers).
Plastics and glass can absorb higher wavelength UV light (below about 340 -- 380 nanometers depending on the type of plastic or glass); quartz prisms pass more UV light.
Oxygen in the air absorbs UV light below 180 nanometers.
UV light is divided by a prism or a grating in spectrophotometers, which are scientific instruments that study how materials absorb light.
Astronomers use the UV light detectors in satellites and telescopes to find UV absorbing substances such as ozone in the earth's atmosphere and to study light from stars, planets and nebulae.