Yes, it is true, Cat urine glows under black light.
In fact, if you can smell cat urine but can’t find the stain, should use a black light to locate the stain in order to treat it.
It’s actually a good way to look for pet urine or make sure a bathroom or hotel room is really clean. You can use a black light to detect body fluids. 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.