The Nobel Prize in Physics 1930
Sir Venkata Raman
Presentation Speech by Professor H. Pleijel, Chairman of the Nobel Committee for Physics of the Royal Swedish Academy of Sciences, on December 10, 1930
Your Majesty, Your Royal Highnesses, Ladies
and Gentlemen.
The Academy of Sciences, has resolved to award the Nobel Prize in
Physics for 1930 to Sir Venkata Raman for his work on the
scattering of light and for the discovery of the effect named
after him.
The diffusion of light is an optical phenomenon, which has been
known for a long time. A ray of light is not perceptible unless
it strikes the eye directly. If, however, a bundle of rays of
light traverses a medium in which extremely fine dust is present,
the ray of light will scatter to the sides and the path of the
ray through the medium will be discernible from the side. We can
represent the course of events in this way; the small particles
of dust begin to oscillate owing to electric influence from the
ray of light, and they form centres from which light is
disseminated in all directions. The wavelength, or the number of
oscillations per second, in the light thus diffused is here the
same as in the original ray of light. But this effect has
different degrees of strength for light with different
wavelengths. It is stronger for the short wavelengths than for
the long ones, and consequently it is stronger for the blue part
of the spectrum than for the red part. Hence if a ray of light
containing all the colours of the spectrum passes through a
medium, the yellow and the red rays will pass through the medium
without appreciable scattering, whereas the blue rays will be
scattered to the sides. This effect has received the name of the
"Tyndall effect".
Lord Rayleigh, who has made a
study of this effect, has put forward the hypothesis that the
blue colours of the sky and the reddish colouring that is
observed at sunrise and sunset is caused by the diffusion of
light owing to the fine dust or the particles of water in the
atmosphere. The blue light from the sky would thus be
light-scattered to the sides, while the reddish light would be
light that passes through the lower layers of the atmosphere and
which has become impoverished in blue rays owing to scattering.
Later, in 1899, Rayleigh threw out the suggestion that the
phenomenon in question might be due to the fact that the
molecules of air themselves exercised a scattering effect on the
rays of light.
In 1914 Cabannes succeeded in showing experimentally that pure
and dustless gases also have the capacity of scattering rays of
light.
But a closer examination of scattering in different substances in
solid, liquid, or gaseous form showed that the scattered light
did not in certain respects exactly follow the laws which,
according to calculation, should hold good for the Tyndall
effect. The hypothesis which formed the basis of this effect
would seem to involve, amongst other things, that the rays
scattered to the sides were polarized. This, however, did not
prove to be exactly the case.
This divergence from what was to be expected was made the
starting point of a searching study of the nature of scattered
light, in which study Raman was one of those who took an active
part. Raman sought to find the explanation of the anomalies in
asymmetry observed in the molecules. During these studies of his
in the phenomenon of scattering, Raman made, in 1928, the
unexpected and highly surprising discovery that the scattered
light showed not only the radiation that derived from the primary
light but also a radiation that contained other wavelengths,
which were foreign to the primary light.
In order to study more closely the properties of the new rays,
the primary light that was emitted from a powerful mercury lamp
was filtered in such a way as to yield a primary light of one
single wavelength. The light scattered from that ray in a medium
was watched in a spectrograph, in which every wavelength or
frequency produces a line. Here he found that, in addition to the
mercury line chosen, there was obtained a spectrum of new sharp
lines, which appeared in the spectrograph on either side of the
original line. When another mercury line was employed, the same
extra spectrum showed itself round it. Thus, when the primary
light was moved, the new spectrum followed, in such a way that
the frequency distance between the primary line and the new lines
always remained the same.
Raman investigated the universal character of the phenomenon by
using a large number of substances as a scattering medium, and
everywhere found the same effect.
The explanation of this phenomenon, which has received the name
of the "Raman effect" after its discoverer, has been found by
Raman himself, with the help of the modern conception of the
nature of light. According to that conception, light cannot be
emitted from or absorbed by material otherwise than in the form
of definite amounts of energy or what are known as "light
quanta". Thus the energy of light would possess a kind of atomic
character. A quantum of light is proportionate to the frequency
of rays of light, so that in the case of a frequency twice as
great, the quanta of the rays of light will also be twice as
great.
In order to illustrate the conditions when an atom emits or
absorbs light energy, we can, according to Bohr, picture to ourselves the atom as
consisting of a nucleus, charged with positive electricity round
which negative electrons rotate in circular paths at various
distances from the centre. The path of every such electron
possesses a certain energy, which is different for different
distances from the central body.
Only certain paths are stable. When the electron moves in such a
path, no energy is emitted. When, on the other hand, an electron
falls from a path with higher energy to one with lower energy -
that is to say, from an outer path to an inner path - light is
emitted with a frequency that is characteristic of these two
paths, and the energy of radiation consists of a quantum of
light. Thus the atom can give rise to as many frequencies as the
number of different transitions between the stable paths. There
is a line in the spectrum corresponding to each frequency.
An incoming radiation cannot be absorbed by the atom unless its
light quantum is identical with one of the light quanta that the
atom can emit.
Now the Raman effect seems to conflict with this law. The
positions of the Raman-lines in the spectrum do not correspond,
in point of fact, with the frequencies of the atom itself, and
they move with the activating ray. Raman has explained this
apparent contradiction and the coming into existence of the lines
by the effect of combination between the quantum of light coming
from without and the quanta of light that are released or bound
in the atom. If the atom, at the same time as it receives from
without a quantum of light, emits a quantum of light of a
different magnitude, and if the difference between these two
quanta is identical with the quantum of light which is bound or
released when an electron passes from one path to another, the
quantum of light coming from without is absorbed. In that case
the atom will emit an extra frequency, which either will be the
sum of or the difference between the activating ray and a
frequency in the atom itself. In this case these new lines group
themselves round the incoming primary frequency on either side of
it, and the distance between the activating frequency and the
nearest Raman-lines will be identical with the lowest oscillation
frequencies of the atom or with its ultrared spectrum. What has
been said as to the atom and its oscillations also holds good of
the molecule.
In this way we get the ultrared spectrum moved up to the spectral
line of the activating light. The discovery of the Raman-line has
proved to be of extraordinarily great importance for our
knowledge of the structure of molecules.
So far, indeed, there have been all but insuperable difficulties
in the way of studying these ultrared oscillations, because that
part of the spectrum lies so far away from the region where the
photographic plate is sensitive. Raman's discovery has now
overcome these difficulties, and the way has been opened for the
investigation of the oscillations of the nucleus of the
molecules. We choose the primary ray within that range of
frequency where the photographic plate is sensitive. The ultrared
spectrum, in the form of the Raman-lines, is moved up to that
region and, in consequence of that, exact measurements of its
lines can be effected.
In the same way the ultraviolet spectrum can be investigated with
the help of the Raman effect. Thus we have obtained a simple and
exact method for the investigation of the entire sphere of
oscillation of the molecules.
Raman himself and his fellow-workers have, during the years that
have elapsed since the discovery was made, investigated the
frequencies in a large number of substances in a solid, liquid,
and gaseous state. Investigations have been made as to whether
different conditions of aggregation affect atoms and molecules,
and the molecular conditions in electrolytic dissociation and the
ultrared absorption spectrum of crystals have been studied.
Thus the Raman effect has already yielded important results
concerning the chemical constitution of substances; and it is to
foresee that the extremely valuable tool that the Raman effect
has placed in our hands will in the immediate future bring with
it a deepening of our knowledge of the structure of matter.
Sir Venkata Raman. The Royal Academy of
Sciences has awarded you the Nobel Prize in Physics for your
eminent researches on the diffusion of gases and for your
discovery of the effect that bears your name. The Raman effect
has opened new routes to our knowledge of the structure of matter
and has already given most important results.
I now ask you to receive the prize from the hands of His
Majesty.
From Les Prix Nobel en 1930, Editor Carl Gustaf Santesson, [Nobel Foundation], Stockholm, 1931
Copyright © The Nobel Foundation 1930
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