Although pets can be caught in the act of an accident, if you're not home, how do you know if that couch smells a little pee-y because a potty break took place . . . or due to your overactive imagination fearing that very occurrence? While you may most associate black lights with those velvety, fluorescent posters, the special ultraviolet rays can reveal urine, vomit, and other substances not visible to the naked eye. When held a couple of feet above carpets, sofas, chairs, or sheets, a black light can reveal a yellow-greenish color if stains are present.