Ultraviolet light is super useful in tanning people who want their skin to look like leather and for killing vampires, but it's not something you can see or feel. So how did people figure out it existed, in the first place? An ingenious — and quite simple — experiment.
Light, though pretty thoroughly abundant when coming from the sun, is a hard thing to get hold of. It travels faster than anything else in the universe, it can't be boxed or bottled, and it can be tough to produce on cue. Because it's so insubstantial, for thousands of years people only managed to pick up a few frequencies, never knowing how much they were missing. It was only through an accidental discovery that anyone realized that there was more to see. During an experiment to test the different temperatures of colored light, Frederick William Herschel channeled sunlight through a prism, splitting it into a rainbow, and then let the colors fall on different thermometers. He noticed the greatest increase in temperature was where the light didn't seem to shine, just beyond the red line of the rainbow. Suddenly, people knew that there was a kind of light that couldn't be seen — infrared.
Johann Ritter read about this discovery and speculated that there might also be invisible light on the other end of the spectrum, beyond the blue lines of the rainbow. Unfortunately for him, he couldn't pull Herschel's thermometer trick, because the temperature increased toward the red end, not the blue end of the spectrum. He was, essentially, hunting something that he could neither see nor feel with any technology available to him.
But that didn't mean that nothing couldn't see or feel it. With a lot of research, Ritter eventually found silver chloride. Silver chloride tarnished and turned black when exposed to sunlight. A little experimentation showed Ritter that it responded more vigorously to the blue end of the spectrum. He used Heschel's prism trick and put strips of silver chloride in each of the separated colors of light. The red tarnished a bit, the orange a bit more, and the violet and blue end of the light blackened the silver chloride impressively. He then tried the whole thing again, but with silver chloride strips out beyond where the violet light fell. Something he couldn't sense made the silver chloride darken more dramatically than any strip under visible light. He'd discovered a new kind of light - one that had always been there but that no one could ever see. Ritter took Herschel's naming conventions, and called it "ultraviolet."
Science can explain things that we already know. It can locate new planets, or new horizons. But I think it's at its coolest when it shows us entirely new dimensions to the world we already know. It's since been discovered that plants and animals can sense ultraviolet light, even if we can't. Herschel and Ritter, together, developed a way to show us that the world was a great deal bigger than anything we see or even sense.