The Raman Effect, It's Theory and Applications

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The Raman effect deals with change of wavelength after scattering of light, which passes through a medium

Generally research in physics is associated with the developed nations. But in 1928 an Indian scientist from Bangalore, propounded a new theory in spectroscopy that has come to be known as the Raman Effect.

The scientist was Sir Chandrasekhar Venkat Raman (he was knighted for this discovery), propounded a new theory on scattering of light. Subsequently he won the Nobel Prize for physics in 1930.

Raman after intensive study at his laboratory in Bangalore concluded that when a light beam travels through a medium, the beam is deflected by the molecules. But more important he observed that a small part of the emerging light beam after deflection by the molecules had a different wave length from the original beam. In other words the wavelength of light after passing through a medium and being deflected by the molecules had a different wavelength.

This change in wavelength of the light beam is known as the Raman Effect and forms an important part of spectroscopy. The limiting factor for this was that the light had to pass from a dust free medium. He also observed that the entire beam did not have its wavelength changed, but only a small part. A simple schematic diagram below will illustrate the Raman Effect

As can be seen from the diagram the incident light after deflection by the molecules is scattered. Some parts of this scattered light have a different wavelength from the original beam.

The original beam may consist of particles or photons. This beam will have a frequency and on striking a surface it gets scattered. Frequency changes were observed by Raman in some parts of the scattered light.

Generally it is seen that the Raman Effect is feeble in liquids, but it is feebler in gases. Hence the Raman Effect is studied in liquids and solids. Gases have low molecular concentration and scattering of light is dependent on striking molecules. The Raman Effect is low in gaseous medium.

The science that deals with this change of frequency is referred to as Raman spectroscopy. This discovery at that time appeared of not much use, but now the Raman Effect has tremendous use in a variety of fields.

A development is the hand held scanners called Raman scanners, which weigh just about 300gms. These are used by US narcotics squads and airports to detect drugs. Security experts have concluded that Raman Scanners can also be used to detect explosives being carried by terrorists. Safety inspectors are already using Raman scanners to detect hazardous chemicals and gases. It is also finding use in forensic work.

The principle of these scanners is simple. They detect the molecular structure of the object after the light is beamed. The change of frequency reveals the molecular structure of the scanned object. This is the Raman Effect. The results are amplified by lasers to arrive at a conclusion. Thus what appeared an innocuous discovery has assumed great importance.

Raman scanning can take just 20 seconds to detect drugs and explosives. Narcotics police in the United States are now using programmed Raman scanners to detect as many as 100 drugs. Whether it is at airports or the scene of a crime, police and security experts use Raman Scanners to detect drugs that include amphetamines, cocaine, crack cocaine, or heroin, the Raman Scanner can detect these illegal drugs. Security experts can now detect explosives that include nitroglycerine and RDX.

Research is being done at Universities like Stanford to find out if Raman Scanners can be used to detect cancer. Other companies are using the Raman Scanner as a way to detect bacteria and other pathogens in hospitals.


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