Meghnad Saha, a great Indian scientist and institution builder

Meghnad Saha: Great Indian Scientist and his Useful Equation

Meghnad Saha: A Short Biography

Family

Meghnad Saha was born on October 6, 1893, in Sheoratoli, Dacca District, East Bengal (now Bangladesh). He was the fifth of eight children born to Jagannath Saha, a poor shopkeeper, and his wife, Bhubaneshwari Devi.

His elder brother failed in high school, so his father decided that Meghnad and his brother should work selling groceries. However, Meghnad’s mother and uncle intervened and allowed him to continue his high school education. In 1905 he went to the city of Dacca where, in preparation for college, he attended the Government Collegiate School with a full scholarship.

Nationalist Movement

It was an eventful year in the history of India: George Nathaniel Curzon partitioned Bengal into Muslim-majority East Bengal (which included the Dacca district) and Hindu-majority West Bengal. That event sparked off a nationalist movement manifested by protests all over Bengal.

When the British lieutenant governor of Eastern Bengal and Assam, Joseph Bampfylde Fuller, visited Dacca, the citizens there organized a boycott. All students who participated in the protests—Saha among them—were expelled.

As a consequence, he lost his scholarship; his brothers helped him survive. In 1911, Saha graduated from Dacca College. He ranked first in physics and mathematics but third in the whole intermediate science examination. He then joined Presidency College at the University of Calcutta, where he worked toward a bachelor’s of science degree in mixed mathematics, which he received in 1913.

Presidency College

Saha graduated from Presidency College, Kolkata, with an M.Sc. in Mixed Mathematics in 1915. Saha and Satyendra Nath Bose were classmates at Presidency College, whose faculty included the well-known chemist and entrepreneur Prafulla Chandra Ray, internationally acclaimed physicist cum plant physiologist Jagadish Chandra Bose, and famous mathematician Devendra Nath Mallik.

Those mentors at Presidency inspired Saha to use science as a tool to promote nationalism and revive ancient India’s intellectual past. Both in the B.Sc. Examination (1913) with Honours in Mathematics and the M.Sc. (Applied Mathematics) Examination (1915), Saha had second place, the first position going to S. N. Bose.

Indian Financial Service

He intended at one time to take the competitive examination for the Indian Finance Service but was not granted permission by the Government. He resolved to devote himself to study and research in applied mathematics and physics.

To support himself and his younger brother staying with him, he for a few months took to private tuitions of which at one time he was doing as many as three in different parts of Calcutta, covering the long distances on a bicycle.

Calcutta University

In 1916 the Calcutta University under the dynamic leadership of its Vice-Chancellor Asutosh Mukerjee, a Judge of the High Court, opened a new University College of Science for post-graduate studies and research— this was made possible because of the magnificent donations of two eminent lawyers of Calcutta, Tarak Nath Palit, and Rash Behari Ghosh.

Saha Saha and S. N. Bose were appointed lecturers in the Department of Mathematics with Dr. Ganesh Prasad as Professor. He, however, found it irksome to get on with the Professor of Mathematics, and in 1917 he (with S. N. Bose) was transferred to the Department of Physics. About a year later C. V. Raman joined the Department as a Palit Professor of Physics. Saha was married in June 1918 to Shrimati Radha Rani Saha.

World War 1

At that time, European scientific literature was limited, and few advanced books were available in Calcutta (now Kolkata) libraries due to World War I. Fortunately, Saha got help from Paul Johannes Brühl, an Austrian scientist at the Bengal Engineering College who provided him with an English translation of the Treatise on Thermodynamics by Max Planck and Die theoretischen und experimentellen Grundlagen des neuen Wärmesatzes (The New Heat Theorem: Its Foundation in Theory and Experiment) by Walther Nernst.

In addition, Saha read A History of Hindu Chemistry from the Earliest Times to the Middle of the Sixteenth Century A. D., written by his mentor Ray, and also familiarized himself with papers by Niels Bohr and Arnold Sommerfeld on the quantum theory of the atom.

Meghnad Saha.
Source – https://commons.wikimedia.org/wiki/File:Dr-Meghnad-Saha.jpg, Licensing – Public Domain

Europe

In 1919 Saha was awarded the Premchand Roychand Scholarship at Calcutta University, and this made it possible for him to spend some two years in Europe. He first went to London and spent about five months in the laboratory of Professor A. Fowler.

Later he moved to W. Nernst’s laboratory in Berlin and did some experimental work on the conductivity of heated cesium vapor to seek an experimental verification of the theory of thermal ionization. In November 1921 Saha returned from Europe and joined the University of Calcutta as Khaira Professor of Physics, a new chair created from the endowment of Kumar Gurprasad Singh of Khaira.

Professor

In 1923 Saha left Calcutta to take up the appointment of Professor and Head of the Physics Department at the University of Allahabad. He held this appointment for 15 years. In 1926 Saha presided at the Physics and Mathematics Section of the Indian Science Congress Association.

Fellow of the Royal Society

In 1927 at the age of thirty-four Saha was elected as a Fellow of the Royal Society. The United Provinces Government sanctioned a personal annual grant of Rs. 5000 for his research work. In 1927, at the invitation of the Italian Government, Saha attended the Volta Centenary Celebration held at Como. He later joined a total solar eclipse expedition to Ringebu (68° N) led by L. Vegard of Oslo University.

In 1936 he was elected to an overseas fellowship of the Carnegie Trust of the British Empire, and he visited Germany, England, and the United States and spent about two months with H. Shapley at the Harvard College Observatory.

In 1938, Saha was offered the Palit Chair in Physics at the University of Calcutta, vacated some years earlier by C. V. Raman who had gone to Bangalore as Director of the Indian Institute of Science. Saha accepted the offer and left Allahabad.

Member of Parliament

He occupied the Palit Chair for fifteen years, retiring in 1953 at the age of 60. In 1951 Saha was elected, as an independent candidate, to the Indian Parliament from the North-West Calcutta constituency. On 16 February 1956, on his way to the Office of the Planning Commission in New Delhi, Saha succumbed to a sudden heart attack at the age of 62.


What are the scientific contributions of Meghnad Saha?

Saha translated Albert Einstein’s Papers

Seated (L to R): Meghnad Saha, Jagadish Chandra Bose, Jnan Chandra Ghosh.
Standing (L to R): Snehamoy Dutt, Satyendranath Bose, Debendra Mohan Bose, N R Sen, Jnanendra Nath Mukherjee, N C Nag.
Source – http://newweb.bose.res.in/Prof.S.N.Bose-Archive/#Photographs, Licensing – Public Domain

Shortly after the end of the First World War there was announced the momentous discovery of the deflection of starlight by the gravitational field of the sun, confirming Einstein’s theory of general relativity. Saha got deeply interested in the relativity theory. He, jointly with S. N. Bose, prepared an English translation of Einstein’s papers, later published in the form of a book by the University of Calcutta.


Saha’s First Papers

The study of relativity led Saha to some investigations in electromagnetic theory and his first original paper entitled ‘On Maxwell’s stresses’ appeared in the Philosophical Magazine in 1917. Saha published a paper on the limit of interference in the Fabry-Perot interferometer in 1917.

He published a paper on a new theorem in elasticity in 1918. This was followed by papers on the dynamics of the electron. He derived, on the basis of the special theory of relativity, the Lienard-Wiechert potential due to a point charge.

During these years he also worked on radiation pressure, and in 1918 (with S. Chakravorti) he published in the Journal of the Royal Asiatic Society of Bengal(Calcutta) a paper on the measurement of the pressure of light, using a resonance method. In 1918 on the basis of his work in the field of electromagnetic theory and radiation pressure, Saha was awarded the D.Sc. degree of Calcutta University.


Saha’s work on Selective Radiation

A comet with its tail.
Credit – NASA, Source – https://commons.m.wikimedia.org/wiki/File:Comet_Diagram_text_stripped.png, Licensing – Public Domain

Saha’s first work in astrophysics

Saha’s first work in astrophysics, and it was a very important one, was the formulation of the concept of selective radiation, and its role in the relative distribution of the elements in the solar atmosphere. In 1919, Saha began examining the contradictions that emerge when the classical theory of light is used to explain such astrophysical phenomena as solar prominences, the corona, and particularly the constant changes that keep a comet’s tail always pointing away from the Sun.

Maxwell’s result

He was aware of James Clerk Maxwell’s result that the pressure light exerts on material bodies is equal to the intensity divided by the speed of light. But he also recognized that Maxwell’s result applied only to obstacles whose dimensions were larger than the wavelength of light.

Because atoms were much smaller than light waves, it followed that they experienced no radiation pressure and, therefore, the mystery of the comet’s tail pointing away from the Sun remained unanswered.

Saha’s Explanation

Saha explained the cometary phenomenon by invoking the energy and momentum of light quanta and noting that pressure is exerted only if the atom is capable of absorbing the radiation. He observed, for example, that in absorbing a pulse of light corresponding to the hydrogen α line, a hydrogen atom would receive an impulsive velocity kick of 60 cm/s.

In subsequent work he remarked, Suppose a continuous spectrum from a bright background passes through a layer of gas. Then the gaseous atoms will be acted upon by only those pulses of light in the continuous spectrum, which the gas is itself capable of emitting and absorbing.

Sodium Spectra

If, for example, the gas is composed of Sodium atoms; then only radiant energy contained in the spectral regions about the D1, D2-lines, and sometimes the other lines of the principal series will act upon the Na-atoms. The remaining part of the continuous light will be without action on the Na-atoms. Regarded from this point of view, the theory may properly be called the theory of Selective Radiative Pressure. Saha did not pursue the subject of selective radiation pressure further.

Work on Compton Effect

It was raised to an entirely new level by E. A. Milne. Saha anticipated the Compton effect by several years with his invocation of a light pulse with frequency ν and energy giving an impulsive kick to an atom that absorbs it. Many physicists in Europe were skeptical about the light quantum before the Compton effect, whose discovery is often seen as a seminal event in physics. Saha, however, effectively used the quantum to explain astrophysical phenomena such as comet tails.


Saha’s contribution to the Theory of High-Temperature Ionization


The Sun was imaged by NASA’s Solar Dynamics Observatory (SDO). This is a false-color image of the Sun observed in the extreme ultraviolet region of the spectrum.
Image Credit – NASA/SDO (AIA), Source – http://sdo.gsfc.nasa.gov/assets/img/browse/2010/08/19/20100819_003221_4096_0304.jpg, Licensing – Public Domain

Saha’s Greatest Contribution

Saha’s greatest contribution is, undoubtedly, the theory of high-temperature ionization and its application to stellar atmospheres. The equation that goes by his name was first given in the paper ‘On ionization in the solar chromosphere’, published in the Philosophical Magazine in October 1920. Using the language of physical chemistry he called it the ‘equation of the reaction-isobar for ionization’.

Calcium Atoms

He regarded the ionization of calcium atoms as taking place according to the following scheme, familiar in physical chemistry,

 where Ca is the normal atom of calcium (in the vapor state), Ca+ is an

atom that has lost one electron, and U is the quantity of energy liberated in the process. The quantity considered, is 1 g atom . . . To calculate the “Reaction isobar” K, let us assume that P is the total pressure, and a fraction x of the Ca-atom is ionized. Then we have

This is the equation of the “reaction-isobar” which is throughout employed for calculating the “electron-affinity” of the ionized atom.’ The value 6.5 in the above expression is the value of the chemical constant obtained from the Sackur-Tetrode-Stern relation.

A new epoch in astrophysics

The theory of thermal ionization introduced a new epoch in astrophysics by providing for the first time, on the basis of simple thermodynamic considerations and elementary concepts of the quantum theory, a straightforward interpretation of the different classes of stellar spectra in terms of the physical conditions (temperature and to a lesser extent pressure) prevailing in the stellar atmospheres.

Using this theory, Saha not only was able to explain the absence in the solar spectrum of the lines of Rb and Cs because of the low ionization potential of these elements but also to predict that their resonance lines were likely to be observed in the relatively cooler regions of the sunspots.


Saha’s contribution to Solar Physics

Solar radiation spectrum for direct light at both the top of the Earth’s atmosphere (represented by yellow area) and at sea level (red area). The sun produces light with a distribution similar to what would be expected from a 5778 K (5505 °C) blackbody, which is approximately the sun’s surface temperature. Image Credit – Nick84, Source – https://commons.m.wikimedia.org/wiki/File:Solar_spectrum_ita.svg, Licensing – Creative Commons Attribution-Share Alike 3.0 Unported

Radiation pressure

Saha used the quantum theory of light and Bohr’s atomic theory to show that the repulsive forces experienced by stellar atoms could indeed be associated with radiation pressure. His theory predicted the spectral lines of some elements that would be found only in relatively low-temperature regions of the Sun.

Puzzling aspects of the spectroscopic data

To explain some of the puzzling aspects of the spectroscopic data of the stars, notably the Sun, Saha came up with the bold suggestion that these data reflect different stages of excitation and extent of ionization of elements “under the stimulus prevailing in the star”, rather than major compositional differences among the stars.

He then went on to derive a formal relationship between temperature, pressure, ionization potential, and relative abundances of different ionization states of an element. It was a remarkably imaginative step that combined the knowledge of the atomic structure in the new subject of quantum mechanics (the Bohr model) and statistical thermodynamics.

The success of Saha’s Theory

The immediate success of Saha’s theory and also some of his predictions led to intense synergistic activities within the Astronomy-Astrophysics community during the 1920s and 30s to further refine the theory and determine the ionization potentials of elements and their spectral characteristics. The ionization theory had a profound impact on the development of the subject of Stellar Astrophysics.

Augmented by subsequent experimental data on thermal ionization and theoretical refinements by Saha and others, such as Ralph Fowler and Edward Milne, it led to a comprehensive understanding of the spectroscopic data of stars in terms of temperature, pressure, and chemical conditions of their atmospheres.

And beyond, it also fixes the epoch or thermal condition when the universe became transparent to radiation and has been applied to a variety of problems in physics and chemistry such as electrical conductivity of flames, formation of an electrical arc, formation of the ionosphere, etc.

Among the twelve great discoveries in the study of stars

This theory was listed by Sir Arthur Eddington in the 14th edition of the Encyclopedia Britannica in 1927 among the twelve great discoveries in the study of stars since 1596. Saha had developed the theory of selective effects of radiation pressure on atoms that could counteract the effect of gravity on the distribution of atoms in the stellar atmospheres; it helped explain such anomalous features as the presence of calcium at greater heights than a much lighter element, hydrogen.

Material that had been hovering in the sun’s atmosphere, the corona, erupted out into space. This phenomenon is known as Coronal Mass Ejection (CME). Image Credit – NASA Goddard Space Flight Center, Source – https://www.flickr.com/photos/[email protected]/7931831962, Licensing – Creative Commons Attribution 2.0 Generic

Prof. Milne of Oxford University

Unfortunately, however, Saha was unable to get the full paper published in the Astrophysical Journal for his inability to pay the page charge that was mandatory at that time because of severe financial constraints in the post-world war period.

The credit for this seminal contribution goes to Prof. Milne of Oxford University who seems to have taken the cue from a short paper by Saha in Nature, and with due acknowledgment, which, unfortunately, did not catch much attention in the Astrophysics community. His work in ionization theory had a long-lasting impact on astrophysics because it changed astronomical spectroscopy from a qualitative tool for the classification of stars to a precise technique for quantitative measurements.

Stratospheric Astrophysical Observatory

In a paper ‘On a stratospheric astrophysical observatory’ which appeared in the Records of Harvard College Observatory, Saha made what at the time (1936) was a very ambitious plea of photographing the solar spectrum at a height of some 50 kilometers, well above the ozone layer; and he pointed out the enormous gain that would accrue for astrophysics.

In this paper, as also in that on the action of ultraviolet sunlight upon the upper atmosphere published in the Proceedings of the Royal Society (1937) and in his Presidential Address to the National Institute of Sciences of India given at Lahore in 1938, he discussed the possibility that the ultra-violet radiation from the sun may be several orders of magnitude above that corresponding to a black-body at about 6500 °K.

Solar Spectrum

He wrote that this may possibly be due to the fact that the UV spectrum of the sun may consist of a continuous background of faint light on which are superposed emission lines of Li, He, He+, Fe+, and other elements which are represented in the visible range by lines of subordinate series, or by patches of ultra-violet continuous light leaking through the solar atmosphere from a much hotter region inside the photosphere.

To account for the Edlen lines in the solar Corona, Saha advanced the somewhat quaint hypothesis that the highly charged ions necessary for the emission are produced as a result of some form of nuclear fission occurring in the sun’s outer atmosphere — he suggested tri- or quadrifission of U or Th. The problem of the emission of radio waves from the sun and other stellar bodies also engaged his attention, and he discussed the likely role of the magnetic field and the hyperfine-structure level-splitting of the ground state of the H-atom.


The Scientific Leadership of Meghnad Saha

Stamp of the Indian astrophysicist Meghnad Saha.
Image Credit – Indian Government, Source – https://commons.m.wikimedia.org/wiki/File:Stamps_of_Indian_Bengali_astrophysicist_Meghnad_Saha.jpg, Licensing – Government Open Data License – India (GODL)

Saha was greatly responsible for the establishment of the National Academy of Sciences at Allahabad and the National Institute of Sciences at Calcutta (headquarters transferred to Delhi in 1946). Saha was the first President (1932-34) of the National Academy of Sciences at Allahabad.

Saha was the General President of the twenty-first annual session of the Indian Science Congress Association held in Bombay in January 1934. The monthly journal Science and Culture, published by the Indian Science News Association (Calcutta), was started by Saha in 1934.

Saha was the second President (1937-39) of the National Institute of Sciences which was renamed the Indian National Science Academy (INSA) in 1970. In 1938, after Saha occupied the Palit Chair in Physics, he spent much of his time in the administration of the laboratories, building a new Institute of Nuclear Physics, and reorganizing and expansion of the laboratories of the Indian Association for the Cultivation of Science.

In 1943 the Bengal Government appointed the Damodar Flood Enquiry Committee. Saha was a member of the Committee, and his contribution was widely recognized as a vital one. The establishment of the River Research Institute in Haringhata, Kolkata was the result of Saha’s initiative and efforts.

Saha was President of the Royal Asiatic Society of Bengal (now the Asiatic Society) from 1944-to 46. He was elected the Honorary Secretary in 1944 and the President of the Indian Association for the Cultivation of Science in 1946.  He was the Indian delegate to the 220th Anniversary Celebrations of the Soviet Academy of Sciences in 1945.

Saha was a member of the University Education Commission appointed by the Government of India in 1948 under the chairmanship of Dr. S. Radhakrishnan. Saha saw the need for an advanced institution in the emerging field of nuclear physics and through his efforts, the Institute of Nuclear Physics in Calcutta was founded in April 1948 (and formally opened in 1950).

Saha was elected Honorary Director of the Institute for life. After his death, the institute was named after him.  In 1952 the post of a full-time Director of the Laboratories of the Indian Association for the Cultivation of Science was created and the choice of the first Director obviously fell on Saha. He accepted the appointment in 1953 on retirement from the Palit Chair of the University which he held for fifteen years.

He was Director of the Laboratories of the Association and Honorary Director of the Institute of Nuclear Physics at the time of his death in 1956. Saha from the very beginning was closely associated with the work of the Council of Scientific and Industrial Research established in 1942. He was a member of the Governing Body and was for many years chairman of the Atmospheric Research Committee, and chairman or member of several other research and planning committees of the CSIR.

He was closely connected with the planning and establishment of the Central Glass and Ceramics Research Institute at Calcutta—one of the national laboratories under the CSIR – and was for many years chairman of its Advisory Committee. He was the chairman of the Indian Calendar Reform Committee, appointed by the CSIR in 1953.


What are some of the other important contributions of Meghnad Saha?

Construction of Damodar River Valley Project.

In collaboration with B. D. Nag Chaudhuri, Saha investigated the problem of the geological age of some of the Indian rocks. Saha stressed the need for a River Research Laboratory to make a scientific study of the complex problems involved in flood control and river utilization generally.

He was one of the first in India to realize the great importance of this subject. He had personal experience of some of the catastrophic floods in the Damodar Valley area (Bengal), and he often took an active part in relief measures organized to deal with them.

The writings of Saha on flood control and harnessing the water of the rivers were responsible for creating public awareness of these problems. He was an active member of the National Planning Committee appointed by the Indian National Congress in 1938.

After the partition of India in 1947, Saha, who himself was from East Bengal, gave a substantial part of his time and energy to the massive human and economic problem of the ‘refugees’ from East Bengal (now Bangladesh). Saha was chairman of the power and fuels sub-committee and a member of the sub-committee on river training and irrigation.


What are the Awards and Honors conferred upon Meghnad Saha?

Meghnad Saha received seven nominations for the Nobel Prize for his ionization theory.


References

  1. Banerjee, S. (2016). Meghnad Saha: Physicist and nationalist. Physics Today69(8), 38-44. doi: 10.1063/pt.3.3267
  2. Bose, D. M. (1965). Meghnad Saha Memorial Lecture. Lecture, National Institute of Sciences of India.
  3. Collected Scientific Papers of Meghnad Saha. (1969). Retrieved 16 February 2021, from http://www.saha.ac.in/web/library-digitized-doc/library-digitized-doc-mn-saha-s-papers
  4. Ganguly, J. (2019). Meghnad Saha – His Science and Persona through Selected Letters and Writings. New Delhi: Indian National Science Academy. ISBN: 978-81-939482-5-5
  5. Kean, S. (2017). A Forgotten Star. Retrieved 16 February 2021, from https://www.sciencehistory.org/distillations/magazine/a-forgotten-star
  6. Kothari, D. S. (1960). Meghnad Saha, 1893-1956. Biographical Memoirs Of Fellows Of The Royal Society5, 216-236. doi: 10.1098/rsbm.1960.0017

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