How does UV light reveal bodily fluids



We’ve all seen it in movies and stuff but how does it work?

In: Chemistry

IIRC in reality it doesn’t work like that (Edit: The only thing I found was seminal fluid was naturally florescent, so there is some exception). They need to first spray a chemical called Luminol which reacts with the iron in blood causing it to be luminescent under black light. Not sure if there are other chemicals that are able to detect other bodily fluids, but this is how they detect blood.

Basically, when a photon uv particle hits a phosphoric material (like pee or sweat) it excites the electrons in the phosphorus. When the electrons move back to their original position, it’s releases energy which we see as light 🙂

UV light causes energy to “go in” to a molecule, if it has the capacity for it, and then that molecule “shines” the light back at lower energy (visible light). What kind of molecules have this property? Anything that has double bonds (high energy bonds) in it, especially if it has a lot of them. A lot of organic molecules just happen to be like this.

The case with blood and Luminol is a reaction that creates a molecule that does this, and phosphorus has high energy bonds with other atoms and also exists in bodily fluids. I’m sure there’s more to this, but that’s my understanding from way back in college organic chemistry in the simplest way I could describe it.

Imagine throwing a ball onto a slanted roof of a two story house. First the ball has to have enough energy to reach the roof, but once it does it can get there, and maybe bounce around a bit. Eventually however the ball will fall off and hit the ground making a sound when it does. Sometimes however the ball will fall onto the garage, and when it does it makes a different sound. This is known as fluorescence.

UV photons are shorter than visible or infrared photons so therefore have more energy and can excite a molecule to a higher energy level, like our roof above. Some of these excited molecules will randomly release light to a different energy level between the roof and the ground level, like our garage case above. This is fluorescence.

The fluorescent photon will always have less energy than the initial photon which is why UV can produce visible light, since visible is less energetic than UV.

Most things will fluoresce given the right conditions, but this can be difficult to achieve. Some well known example in nature are scorpions and some algae