Raman effect (elementary idea)
When light scatters from a transparent substance, a small part comes out with a changed frequency. This is the Raman effect, found by C. V. Raman in 1928.
What Raman found
Shine light of a single frequency ν on a transparent substance such as benzene or water, and study the scattered light with a spectrometer. Most of it has the same frequency ν. A very small part has slightly lower or slightly higher frequencies. This change of frequency on scattering is the Raman effect, and the spectrum of the scattered light is the Raman spectrum. The energy of light is linked to its frequency by E = hν. So a change in frequency means the light has exchanged energy with the molecules. The size of each shift depends on the substance, so the Raman spectrum works like a fingerprint of its molecules. A similar change was seen earlier by Compton for X-rays. Your NIOS lesson gives the year as 1926, but Raman announced the discovery on 28 February 1928.
Three things that can happen when light meets a molecule
| Case | Energy of the light | Scattered frequency | Line in the spectrum |
|---|---|---|---|
| No energy is exchanged | Unchanged | Same as ν | Unshifted line (strongest) |
| Light gives energy to the molecule | Decreases | Lower than ν | Stokes line |
| Light takes energy from an already excited molecule | Increases | Higher than ν | Anti-Stokes line |
Uses of the Raman effect
- Identifying molecules and studying their structure in chemistry laboratories.
- Checking the purity of medicines without opening or damaging them.
- Testing gemstones and spotting fakes.
- Studying crystals and new materials.
- India celebrates National Science Day on 28 February, the day Raman announced his discovery.
How to draw a Raman spectrum
- Draw a horizontal arrow and label it frequency.
- Draw a tall line in the middle and label it ν (incident frequency, unshifted line).
- To its left, draw a few short lines and label them Stokes lines (lower frequency).
- To its right, draw a few shorter lines and label them anti-Stokes lines (higher frequency).
- Write under the diagram: the shifts are the same on both sides and depend on the substance.
India marks National Science Day every 28 February in honour of Raman's discovery, made in Kolkata in 1928. Today handheld Raman instruments check medicines and identify gemstones without damaging them.
2 marks: “What is the Raman effect? What are Stokes and anti-Stokes lines?” When light is scattered by a transparent substance, a small part of the scattered light has a changed frequency. Lines of lower frequency than the incident light are Stokes lines; lines of higher frequency are anti-Stokes lines.
2 marks: “Using E = hν, explain why some lines in a Raman spectrum have a lower frequency than the incident light.” The light gives part of its energy to the molecule it meets. With less energy, and E = hν, the scattered light has a lower frequency: these are the Stokes lines.
1 marks: “Who discovered the Raman effect, and in which year was he awarded the Nobel Prize for it?” C. V. Raman; Nobel Prize in Physics, 1930.
Stokes sinks, anti-Stokes soars: Stokes lines have a lower frequency, anti-Stokes lines a higher one.