Ultraviolet Light is on the edge of the color spectrum that humans can see. Purple is the lowest wavelength that humans can see, ultraviolet has some of that purple in it as well as light that the human eye cant see.
Ultraviolet refers to the part of the color spectrum that lies below 350nm. The LEDs that we refer to as UV this includes a little above 350nm hence the purple that we can see. On the other end there is infrared above 750nm, sometimes with IR LEDs you can see a red glow, that is the same phenomenom that you see with UV LEDs, just it is at the other end of the visible spectrum(and no where near as cool).
when working with either UV or Infrared LEDs, caution must be used, since there is light that you cant see(and you dont know how much). Just make sure not to overdrive them and shine them into your eyes from a close distance. (IR is most dangerous when it comes to this) Most of consumer LEDs are safe but at least be aware that a danger does exist when your eyes are really close to the LED.
