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C. V. Raman Sticker

Most Pass Free. A Few Pay A Toll. | Physics

\[\begin{gathered} h\nu_s = h\nu_0 - h\nu_{\text{vib}} \\ (\text{Stokes}) \end{gathered}\]
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Shine one colour of light through a liquid and a few photons come out changed. In 1928 C. V. Raman and K. S. Krishnan found a tiny fraction of scattered light shifts colour by a molecular vibration energy. A pixel-art portrait on a 3" × 3" sticker; the physics panel below has the equation, every symbol and the sources.

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The physics

Most light scattered by molecules keeps its colour, but a tiny fraction gives up or takes energy from a molecular vibration and comes out shifted. That Raman shift is a fingerprint of the molecule's vibrations.

The sticker says Most Pass Free. A Few Pay A Toll. It's a fair account of light going through a clear liquid.

Almost all the light a liquid scatters comes out the colour it went in. In early 1928, at 210 Bowbazar Street in Calcutta, C. V. Raman and K. S. Krishnan found the exceptions. Working with carefully dust-free liquids and gases, they saw a feeble scattered glow at new colours and reported it in Nature as a new type of secondary radiation. Their letter is dated 16 February and was published on 31 March.

Here is the toll. A photon can set a molecule vibrating and leave with exactly that much less energy, so it comes out redder. That is the Stokes line on this sticker. More rarely a photon collects energy from a molecule that is already vibrating and comes out bluer, the anti-Stokes line. Only a tiny fraction of scattered photons, very roughly one in a million, pay or collect at all.

Because the shifts are set by the molecule's own vibrations, the shifted light is a fingerprint of whatever it passed through. Raman spectrometers now identify minerals, monitor medicine production and analyse seized drugs through sealed bags, without touching the sample.

Raman received the 1930 Nobel Prize in Physics alone. Krishnan co-signed the discovery. Adolf Smekal had predicted the effect in 1923, and Grigory Landsberg and Leonid Mandelstam saw it in quartz in 1928, at almost the same time. Raman was also the uncle of Subrahmanyan Chandrasekhar, who has a sticker in this set.

Most photons pass free. A few pay a toll.

Equations

\[\begin{gathered} h\nu_s = h\nu_0 - h\nu_{\text{vib}} \\ (\text{Stokes}) \end{gathered}\]
\[\begin{gathered} h\nu_s = h\nu_0 + h\nu_{\text{vib}} \\ (\text{anti-Stokes}) \end{gathered}\]
\[\Delta\tilde\nu = \frac{1}{\lambda_0} - \frac{1}{\lambda_s}\]
SymbolMeaningUnit
\(h\) Planck constant \(\mathrm{J·s}\)
\(\nu_0\) frequency of incident light \(\mathrm{Hz}\)
\(\nu_s\) frequency of scattered light \(\mathrm{Hz}\)
\(\nu_{\text{vib}}\) molecular vibration frequency \(\mathrm{Hz}\)
\(\Delta\tilde\nu\) Raman shift in wavenumber (usually quoted per centimetre) \(\mathrm{m⁻¹, cm⁻¹}\)
\(\lambda_0, \lambda_s\) incident and scattered wavelengths \(\mathrm{m}\)

Sources

  1. Raman & Krishnan (1928) A New Type of Secondary Radiation, Nature 121, 501
  2. NobelPrize.org — The Nobel Prize in Physics 1930, C. V. Raman (facts)
  3. American Chemical Society — The Raman Effect, National Historic Chemical Landmark
  4. Wikipedia — Raman scattering (Smekal 1923; Landsberg and Mandelstam 1928)

Source Raman, C. V.; Krishnan, K. S. (1928). A New Type of Secondary Radiation. Nature 121, 501–502.doi.org/10.1038/121501c0

Field notes

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