When a monochromatic (i.e. single-frequency) beam of light traverses a medium (gas, liquid or solid) the majority of the scattered light will remain at the incident frequency. However, a small proportion of the scattered light will be at changed frequencies, above and below the incident frequency, and this is referred to as the Raman effect. The Raman effect was first observed by Raman and Krishnan (1928) using focused sunlight and filters and relied on the visual observation of colour changes in the scattered light. However, it was not until the advent of continuous wave visible lasers, during the 1960s, that the importance of Raman spectroscopy as a routine analytical technique was realized. Furthermore, the availability of this highly intense monochromatic light source, which could be focused to a narrow waist, allowed the analysis of small volumes of gas, liquid or solid.
