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Most students meet K. S. Krishnan as a name next to C. V. Raman's on a 1928 paper. That is a small part of his story. After the Raman effect, Krishnan built a second career using magnets to look inside crystals, and his methods connect directly to the para- and diamagnetism you study for JEE. This post covers both discoveries and the physics behind them.
Who was K. S. Krishnan?
Kariamanikkam Srinivasa Krishnan was born on 4 December 1898 in the village of Watrap in present-day Tamil Nadu, according to the Royal Society memoir by Kathleen Lonsdale and Homi Bhabha. A profile by Vigyan Prasar, the science communication body of India's Department of Science and Technology, records that he studied at the American College in Madurai and Madras Christian College. While working as a demonstrator in chemistry he ran informal lunch-hour discussions on physics and chemistry that drew students from other colleges. In 1920 he went to Calcutta to work with Raman at the Indian Association for the Cultivation of Science.
What was Krishnan's part in the Raman effect?
The Raman effect is the small fraction of scattered light that comes out at a different frequency from the incoming light, because the molecules exchange energy with it. Raman and Krishnan announced it in a short letter to Nature, A New Type of Secondary Radiation (1928).
The clearest account of Krishnan's role is Raman's own. In his 1930 Nobel lecture, Raman credits Krishnan with examining a great many liquids to show how the optical anisotropy of a molecule depends on its chemical make-up, and says Krishnan took up the search for the new radiation again towards the end of 1927. He then writes that "Krishnan, who very materially assisted me in these investigations", found the effect in several organic vapours. With a larger spectrograph, Krishnan obtained the spectrograms on which lines displaced towards the violet were first definitely established. The Nobel Prize went to Raman alone.
For the effect itself, read our post on C. V. Raman and the Raman effect.
How did magnets reveal the inside of crystals?
In December 1928 Krishnan moved to Dacca University, where Satyendra Nath Bose headed the physics department. There, the Vigyan Prasar profile records, he developed precise methods to measure the magnetic anisotropy of diamagnetic and paramagnetic crystals.
The idea is set out in the first of a series of papers, Investigations on magne-crystallic action. Part I: Diamagnetics (1933), with Guha and Banerjee. Many molecules respond to a magnetic field differently depending on which way they face. When such molecules are packed in a regular way in a crystal, the crystal as a whole has different magnetic susceptibilities in different directions. Comparing the crystal's anisotropy with that of its individual molecules tells you how the molecules are oriented in the crystal. The authors wrote that in certain crystals the orientations could be found more easily and more accurately this way than by X-ray methods.
What do dia- and paramagnetism mean in your syllabus?
The JEE (Main) 2026 syllabus lists "para-, dia- and ferromagnetic substances with examples" and "torque on a magnetic dipole in a uniform magnetic field" in its magnetism unit. Check the bulletin for your exam year, as NTA revises it. In outline:
Diamagnetic materials have a small negative susceptibility. A field induces a weak magnetisation opposite to it, so they are weakly pushed towards weaker field.
Paramagnetic materials have a small positive susceptibility. Their atoms carry permanent magnetic moments that partly line up with the field, so they are weakly pulled towards stronger field.
Ferromagnetic materials, such as iron, have very large positive susceptibility and can stay magnetised.
The common mistake is to think diamagnetism belongs only to a few odd materials. Every substance has a diamagnetic response; in paramagnetic and ferromagnetic materials it is outweighed. Krishnan's crystals added one more layer: susceptibility can also depend on direction. Our post on magnetic materials and hysteresis covers the exam side in more detail.
What came later?
Vigyan Prasar records that Krishnan returned to Calcutta in 1933, moved to Allahabad University in 1942 and became director of the National Physical Laboratory in New Delhi in 1948. He was elected a Fellow of the Royal Society in 1940, knighted in 1946, awarded the Padma Bhushan in 1954 and became the first recipient of the Bhatnagar Memorial Award in 1961. He died in June 1961.
What can a student take from Krishnan's way of working?
Krishnan moved between optics, magnetism and solid-state physics, and used one field to answer questions in another. Two habits are easy to copy. Start a discussion group like his lunch-hour sessions, where anyone can bring a question in physics, chemistry or maths. And when you learn a method in one chapter, ask where else it could work. Magnetic fields and their torques are hard to picture from a textbook figure; the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than to read.
This week, write the definitions of dia-, para- and ferromagnetism from memory, then add one example of each and check them against your NCERT textbook.
