Why is black light purple?

Answer 1

There are two things going on here. The first thing is that if you look at most "black light" bulbs when they're on you'll see they're not actually black, they're purple. When you shine a purple light on a white object, it looks purple. This is the dull purple color that some white objects have under black light. Many white articles of clothing, though, shine a fairly bright purple (in many cases it's more of a bluish color). Underwear is particularly good at this (which may be either embarrassing or exactly the effect you were going for, depending on your personal sense of fashion). In this case, what's happening is: laundering. Many fabrics yellow slightly over time, making them appear dingy. To counteract this, detergent (and sometimes bleach) manufacturers include "whiteners" in their formula. These work not by removing the yellow, but by adding blue... a fluorescent compound is included which raises the intensity in the blue region of the spectrum, and when you mix blue light with yellow light you get what your eye perceives as white. Under "black light" these fluorescent compounds go nuts, and since the visible light level is pretty low, the blue light they emit can be clearly seen, in many cases even if there's another layer of clothing on top of the white ones.

Answer 2

prisms