The exact answer to the question "What body fluid glows red under UV light?" is that no common body fluid consistently glows red under UV light through direct fluorescence. While many body fluids contain fluorescent molecules that allow them to glow under ultraviolet (UV) light, a distinct red glow is not a commonly observed characteristic for these fluids themselves.
Understanding Body Fluid Fluorescence Under UV Light
Body fluids contain various organic compounds that naturally fluoresce, meaning they absorb light at one wavelength (like UV) and emit it at a longer, visible wavelength. This property is widely utilized in forensic science for locating biological evidence. Forensic scientists often use ultraviolet lights at crime scenes to find blood, urine, or semen, which contain these fluorescent molecules.
Common Body Fluids and Their UV Fluorescence
While the specific color of fluorescence can vary based on factors like concentration, substrate, and the exact UV wavelength used, here's a general overview of how common body fluids typically react under UV light:
- Blood: Unlike what the question suggests, blood itself does not typically glow red under UV light. Hemoglobin, a primary component of blood, tends to absorb UV light, often making bloodstains appear dark or black under UV. Any weak fluorescence from other blood components is usually not in the red spectrum. Forensic detection of blood often relies on chemical reagents (like luminol) that react with blood to produce a visible luminescence (often blue or green) through a chemical reaction, not direct fluorescence of the blood itself.
- Urine: Urine commonly fluoresces with a yellowish-white or pale blue hue under UV light. This is primarily due to the presence of compounds like riboflavin (Vitamin B2) and other metabolic byproducts.
- Semen: Semen is known to fluoresce vividly under UV light, typically emitting a strong bluish-white or bright yellow glow. This fluorescence is mainly attributed to the presence of flavins and choline.
- Saliva: Saliva can also fluoresce, usually appearing as a pale blue or greenish-blue glow under UV light.
- Vaginal Fluids: Similar to semen, vaginal fluids can exhibit a bluish-white fluorescence.
Summary of Typical Fluorescence Colors
To provide a clearer overview of the expected colors, consult the table below:
| Body Fluid | Typical UV Fluorescence Appearance |
|---|---|
| Blood | Appears dark (absorbs UV) |
| Urine | Yellowish-white, Pale Blue |
| Semen | Bluish-white, Bright Yellow |
| Saliva | Pale Blue, Greenish-Blue |
| Vaginal Fluids | Bluish-white |
Why Not Red?
The specific chemical composition of fluorescent molecules within body fluids determines the color of light they emit. For most biological fluids, these molecules primarily emit light in the blue, green, or yellow regions of the visible spectrum. Red fluorescence is less common for naturally occurring compounds in body fluids when exposed to typical UV wavelengths used in forensic applications.
