= Raman and Krishnan light-scattering experiment
{c}
<Chandrasekhara Venkata Raman> and <Kariamanikkam Srinivasa Krishnan> jointly reported the effect in 1928. Their letter describes <light> from the <Sun> concentrated by an $18\,\mathrm{cm}$-<aperture>, $230\,\mathrm{cm}$-<focal length> telescope objective and a second <optical lens> onto a liquid purified by repeated <vacuum distillation>. With blue-violet and yellow-green <optical filters> together in the incident beam, the visible track disappeared; moving the yellow-green <optical filter> between liquid and observer restored it. The complementary arrangement rejected unchanged-colour <scattering of light> while admitting newly generated <light>. Persistence after purification and strong <wave polarization> in many liquids argued against ordinary fluorescent contamination.<A New Type of Secondary Radiation by Chandrasekhara Venkata Raman and Kariamanikkam Srinivasa Krishnan>[Raman and Krishnan, A New Type of Secondary Radiation, Nature 121, pp. 501–502; letter reproduced on PDF page 5 at dkasc.ac.in]{ref}
In his 1930 lecture, <Chandrasekhara Venkata Raman> describes a cobalt-glass <optical filter> selecting indigo-violet <light> from the <Sun>, with a separated blue-green band in the scattered <optical spectrum>. Substituting a quartz mercury arc and condenser gave sharp new <spectral lines>. Small Hilger quartz <spectrographs> recorded them photographically; <Kariamanikkam Srinivasa Krishnan> obtained more precise records with a larger instrument and established violet-displaced lines. The <optical prism> disperses <wavelengths> for spatial readout; it does not create the <Raman shift>. The diagram gives functional optical paths rather than a reconstruction of a specific Hilger instrument.<Nobel Lectures, Physics 1922–1941/The Molecular Scattering of Light by Chandrasekhara Venkata Raman>[Nobel Lectures, Physics 1922–1941; Raman, The molecular scattering of light, Nobel lecture, 11 December 1930, printed pp. 271–273, spectroscopic examination and photographic recording; inspected in a reproduction of the complete lecture]{ref}
\Image[/raman-filter-and-prism-apparatus.png]
{title=Complementary <optical filters> and a dispersive readout test different parts of the <Raman scattering> evidence}
{description=Original functional schematic of the filter test and later spectral test. Side collection is unfolded for readability. The <optical prism>, <spectrograph slit> and <photographic plate> indicate dispersion and recording; the sketch supplies no historical distances, exact optical prescription or measured colour intensities.}
{source=raman-filter-and-prism-apparatus.py}
Their May 1928 report strengthens the comparison: filtered mercury illumination of <toluene> produced extra <spectral lines>. Removing the $4358\,\text{Å}$ incident line removed its associated longer-<wavelength> group; a quinine filter removing the $4047$, $4078$ and $4109\,\text{Å}$ group removed a different shifted group while preserving the one associated with $4358\,\text{Å}$. These are actual selective-excitation controls, rather than inferred colours.<The Optical Analogue of the Compton Effect by Chandrasekhara Venkata Raman and Kariamanikkam Srinivasa Krishnan>[Raman and Krishnan, The Optical Analogue of the Compton Effect, Nature 121, p. 711, Fig. 1 description and removal of individual incident-line groups]{ref}
The inference has limits. A complementary-filter test alone establishes newly admitted <wavelengths>, not their precise spacing or a unique microscopic mechanism. <Fluorescence> can also survive purification when intrinsic to the liquid, and <fluorescence> can be polarized. Useful distinctions therefore combine resolved <spectral lines>, excitation dependence, <wave polarization> and sample preparation. A proposed modern extension is to change a single incident <frequency>: a molecular <Raman shift> should follow the same internal <energy> gap, whereas ordinary relaxed molecular <fluorescence> usually retains approximately its emission-band positions. This is a diagnostic under stated conditions, not a universal theorem about every fluorescent material or resonant scattering process. <Raman scattering> need not involve a populated, relaxed electronic <excited state> as ordinary molecular <fluorescence> does; neither a redder photon nor the word Stokes identifies the mechanism by itself.
The 1930 <Nobel Prize in Physics> went to <Chandrasekhara Venkata Raman> alone for his work on <scattering of light> and the discovery of the effect. That award allocation should be distinguished from the joint experimental report with <Kariamanikkam Srinivasa Krishnan>.<The Nobel Prize in Physics 1930 by Nobel Foundation>[Nobel Foundation prize record: sole laureate and award motivation, Physics 1930]{ref}
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