A black light produces wavelengths of ultraviolet light ranging from about 315 to 400 nanometers, which can make some invisible stains visible to the human eye. The ultraviolet waves react with a variety of chemical compounds, including B vitamins, chlorophyl and quinine, causing their molecules to glow in blues, greens and other colors.
Under black light, petroleum jelly has a bright blue glow. People probably aren't normally looking for invisible Vaseline stains, but it can be fun to show kids this effect.
Residue from blood can show up under black light even when the blood has been washed away. Blood stains under ultraviolet light look about 4 times darker than normal, nearly black.
Black light illuminates urine stains, which is useful if you're smelling cat or dog urine in your house but can't pinpoint where the odor is coming from. Urine has a bright yellow or yellow-green glow when illuminated by black light.
Saliva and Semen
Semen and saliva stains also glow as a bright yellow or green-yellow color under ultraviolet light.