Fluorescent highlighter markers are so bright because they are literally fluorescent. When used to describe highlighters, the word "fluorescent" is not a vague term that means "extra bright". Rather, this word is an exact, scientific term indicating that the highlighter ink exhibits fluorescence. Fluorescence is the phenomenon where a material absorbs light of a certain color and then emits light of a different color with a longer wavelength. The most striking type of fluorescence involves the absorption of ultraviolet rays (which humans can't see) and the subsequent emission of light in the visible spectrum (which humans can see). Because humans can't see the original ultraviolet light, a fluorescent object looks like it is glowing mysteriously on its own when it is illuminated only by ultraviolet rays in a dark room . For this reason, ultraviolet lights and fluorescent materials can add an intriguing look to darkened rooms at parties and events. Since highlighters contain fluorescent chemicals, the marks made by highlighters will seem to eerily glow on their own when placed in a dark room with an ultraviolet light (e.g. a "black light").