C. V. RamanMost Pass Free. A Few Pay A Toll.

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.