One of the most popular methods for detecting illegal drugs and unknown substances is Raman spectroscopy. The technique takes advantage of the Raman effect, named after C.V. Raman of Calcutta, India, a professor at Calcutta University who theorized that when light passes through a solid, liquid or gas it scatters in such a way that a very small fraction of the reflected light is a different color from the incident light. Now, more than 70 years after the discovery, which earned Raman a Nobel Prize in physics, the American Chemical Society and the Indian Association for the Cultivation of Science have designated the discovery an international historic chemical landmark. They have erected a plaque at the institute in Calcutta where Raman performed his research.