What exactly is “black light”? You know, that light from purple-glowing bulbs or tubes, that makes some things (especially white things) glow in the dark, but otherwise seems to cast no illumination. Where does it fit on the EM spectrum? How does it make things glow?
It’s ultra-violet (UV).
Black light is a misnomer for ultra-violet light.
Certain pigments glow in visible colors when exposed to UV light. This effect is called Phosphorescence. It’s more common than you might think. Many laundry detergents add phosphoescent agents to their formulas to get your brightly colored shirt glowing in the sunlight, which contains UV light.
Quibble: it’s more usually called fluorescence. The difference (see here or here) is that fluorescence is when something glows in response to UV; phosphorescence when it continues to glow for a while after the source is removed.
But I though UV was invisible to human eyes. Why does the blacklight bulb/tube glow violet?
It glows violet because it’s not perfect. The light it emits peaks in the UV band, but the frequency spread is enough to reach the visible range.
IIRC Florescence refers to the process whereby a material absorbs the energy from light of one wavelength and then re-emits that energy back again but at a different, usually longer wavelength. It is then possible to absorb in the invisible UV spectrum and re-emit in the visible.
Laundry detergents use this property to give ‘whiter-than-white’ clothes. They contain a substance which flouresces so that your clothes appear to reflect back more light because some of the invisible UV is now visible.
Florescent light strips have a florescent coating on the inside of the tube which converts the UV given off by the low pressure mercury vapour tube into visible light. A ‘black light’ is just one of these tubes without the coating. They rely of florescent compounds in white clothes etc. to make the light visible.
Okay, so the UV bulbs you can pick up at the mall glow violet because they’re cheap and don’t worry about some light radiation being emitted out of the UV range. But could you get a black light that doesn’tleak into the visible spectrum?
No. I had an expensive geological blacklight used to help identify minerals via their fluorescence, and it, too, glowed somewhat violet. In theory, you could further coat the tube or bulb with a material which only passes UV, but I’ve never seen this done, presumably because it’s impractical to do so.
The leakage into visible range is down to the emission spectra. UV bulbs are based on mercury vapour discharge, which has a line spectrum with a big peak at 254nm, but with smaller peaks, some in the visible range. You can get long-wave UV lamps, where phosphors absorb this 254nm and re-emit it as longer-wavelength UV, but then you get a broad-spectrum emission whose tail is again in the visible range. View it as a design feature: at least you can see when it’s switched on. 