Cop shows and movies show you an unbelievable range of gizmos that can perform unimaginable tasks - such as bringing into focus a single voice from an otherwise muffled recording or computers that can zoom in on a tiny section of the image on the screen. Most of it is pure fiction. But one seemingly strange chemical that glows when it comes into contact with blood is for real. It's called Luminol.

Luminol is a chemical that glows with a bluish light when it comes in contact with blood. It even reacts with years old traces of blood. An ultra-sensitive chemical, it can detect blood at one part per million. In simple language, this means that if there is even one drop of blood in 999,999 drops of water, luminol will glow. Hence, extensively used for criminal investigations, luminol has helped crack some pretty tricky cases of detection.

Victims of bloody, violent crimes cannot disappear without a trace - that is the basic assumption on which crime scene investigators function. No matter how hard the murderer tries to clean up the blood and dispose of the body, some tell-tale traces of blood will remain. Tiny blood particles can cling to most surfaces for years on end, without ever being seen.

Luminol is effective in revealing these traces. There is a chemical reaction between several chemicals and hemoglobin (a protein in the blood that carries oxygen), that produces light. There is a breakdown of molecules, which then rearrange themselves to form different molecules. In the chemical reaction of luminol, the original molecules (reactants) have more energy than the new molecules (products). The extra light is discarded by the molecules in the form of light photons. This phenomenon is known as chemiluminescence.

Luminol is typically used at a crime scene where bloodshed is suspected to have happened but no traces of blood are visible to the naked eye. The room is darkened and then the chemical is sprayed over a large area. If blood traces are revealed, then investigators videotape or photograph the crime scene.

The tricky thing about luminol is that substances (such as household bleach) other than blood may also cause luminol to glow. Hence, a glow merely reveals to investigators the possibility of blood in the area. An experienced investigator, by observing the speed of the chemical reaction can judge whether the substance was blood or something else. But in order to be absolutely sure that it was blood, other tests need to be run.

Luminol helps to provide clues as to how events may have unfolded or the kind of weapon used. For example, blood splatter patterns of a blunt object versus those of a sharp knife will be different. Sometimes, bloody shoe prints give a lot of clues about the movements of the assailant after the attack. Luminol may also lead investigators to discover new evidence. For instance, if there is a positive reaction to luminol sprayed on a carpet, then removing the rug may reveal blood-soaked floorboards.

Despite being a key investigative tool, luminol has one disadvantage - the chemical reaction can destroy other evidence in the crime scene. Hence, contrary to what a layman might think, luminol is only used after other extensive investigations have failed to reveal enough clues.