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Why Does Urine Glow Under a Black Light?

Element in Urine That Glows

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Urine glows when exposed to black or ultraviolet light.

Urine glows when exposed to black or ultraviolet light.

Anne Helmenstine White phosphorus powder glows green in the presence of oxygen.

White phosphorus powder glows green in the presence of oxygen.

Luc Viatour, Creative Commons License

You can use a black light to detect body fluids. It's actually a good way to look for pet urine or make sure a bathroom or hotel room is really clean. Cat urine, in particular, glows very brightly under ultraviolet light. Urine glows under a black light primarily because it contains the element phosphorus. Phosphorus glows yellowish green in the presence of oxygen, with or without black light, but the light imparts additional energy that make the chemiluminescence easier to see. Urine also contains broken down blood proteins that glow under a black light.

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