"beyond the violet end of the visible spectrum," 1840, from ultra- "beyond" + violet. Ultra-red (1870) was a former name for what now is called infra-red.
Entries linking to ultraviolet
ultra-
word-forming element meaning "beyond" (ultraviolet) or "extremely" (ultramodern), from Latin ultra- from ultra (adv. and prep.) "beyond, on the other side, on the farther side, past, over, across," from PIE *ol-tero-, suffixed form of root *al- "beyond." In common use from early 19c., it appears to have arisen from French political designations. As its own word, a noun meaning "extremist" of various stripes, it is first recorded 1817, from French ultra, shortening of ultra-royaliste "extreme royalist."
violet n.
small wild plant with purplish-blue flowers, c. 1300, from Old French violete (12c.), diminutive of viole "violet," from Latin viola "the violet, a violet color," cognate with Greek ion (see iodine), probably from a pre-Indo-European substrate Mediterranean language. The color sense (late 14c.) developed from the flower.