Black light is electromagnetic radiation composed of ultraviolet rays and visible violet radiation with a wavelength of violet light.
Black light is basically ultraviolet radiation combined with a small percentage of violet-colored visible light . The light emitted by fluorescent lamps, for example, is largely produced by the thin white layer that surrounds it, composed of phosphor salts .
Some phosphorus salts and other oxides can emit visible light when excited by ultraviolet light. This particular phenomenon of luminescence is called phosphorescence (a phenomenon that occurs on the screens of CRT monitors ) and has a quantum origin , occurring thanks to an increase in the energy levels of electrons. As atoms are illuminated, their electrons absorb energy , passing through non – stable energy levels . When the external stimulus ceases, the electrons return to lower energy levels , emitting a weak glow and characteristic of the energy difference of the transitions.
How do we produce black light?
Black light can be easily produced with fluorescent lamps by removing its white layer and tinting it with some black ink , which is capable of absorbing most wavelengths of visible light, but which lets ultraviolet light through .
Uses of black light
This type of light is widely used by forensics to analyze crime scenes, as some physiological fluids , such as semen, saliva and urine, as well as bone and tooth fragments, phosphorize in the presence of black light . Although blood is not phosphorescent , it is common to use luminescent substances , such as luminol , which, when combined with hydrogen peroxide (H 2 O 2 ), produce a slow chemical reaction that is catalyzed by iron , present in blood hemoglobin , speeding up the process and making the smallest traces of blood visible.