Why'd we divide up the UV spectrum?

There’s UVA, UVB, and who knows what else. Was this just for convenience, to break a large category up into arbitrary smaller parts, or is there a scientific basis?

In addition to UVA/UVB/UVC, you’ll also see another scale: Longwave (roughly equivalent to UVA), medium wave (roughly equivalent to UVB), shortwave (roughly equivalent to UVC) and extreme UV (which verges on the soft X-ray range). Shorter wavelengths are more energetic, but less penetrating (they are more easily absorbed) Both systems are, as you noted, largely arbitrary.

UVA is 315-400 nm; UVB is 280-315 nm and UVC 100-280 nm.

The UVA/UVB categories were first used in minerology AFAIK. Fluorescence can be a useful screening tool in commercial geology: e.g. scheelite, a tungsten ore, glows pale blue when pure, but yellowish or white if contaminated with molybdenum, which was difficult to remove commercially. Most “fluorescent minerals” actually fluoresce weakly or not at all when pure, so the color and degree of fluorescence is a useful way to quickly guess at ore impurites

Though minerals respond differently to different wavelengths, IIRC, the original A/B distinction was in the UV tubes. UVA penetrates glass reasonably well, but UVB requires more expensive quartz tubes. The greater expense is worthwhile, because many more minerals fluoresce in the more energetic UVB.

Biologically, the UVB band contains the frequencies that are absorbed most by DNA (peak ~260nm) and proteins (peak ~280nm) and can cause up to 1000x the immediate “sun damage” to skin and the lens of the eye as the same dose of UVA (though UVA my be more important in cataracts than previously thought).

95% of the UV in sunlight is UVA, which isn’t entirely benign, especially with long or repeated exposure. UVA can produce free radicals which can produce UVB-like DNA and protein effects indirectly, as well as depleting cell antioxidants.

UVA is used in tanning beds and the “black lights” used to illuminate posters Tubes that produce the most energetic UV (UVC) are used as germicidal lamps - they cause enough damage to efficiently kill cells.

I meant to mention that sunlight contains almost no UVC (it’s absorbed by the atmosphere), but prolonged sunlight has some germicidal effect from UVB. Grandma knew what she was doing when she sun-dried her laundry on the clothesline.

Wow, that was an awesome response. You’ve blocked my UV ignorance.