system
Given the following post, respond as if you're commenting on Reddit:
human
If UV light is invisible, why do UV LEDs (on blacklights etc.) have that unique violety-blue colour?

Just wondering why you can see that unique colour if UV light is meant to be invisible. Is it actually the colour on the cutoff of the spectrum where it turns into UV?
gpt
You answered your own question, the colour that you see is at the far end of the range of visible light. We define the UV part of the spectrum as being adjacent to the visible light part, but since the definition of the transition is somewhat arbitrary, there's a fuzzy overlap area between visible and UV (and between infrared and visible on the other end).

Having UV lights emit some light within the visible range can be very helpful, because it gives immediate visual feedback that the light is turned on and functioning normally.