VISMAYA – History & Philosophy of Physics – Page 46 – HUMANIZING SCIENCE : PEOPLE, WONDER, IDEAS & TOOLS

Curiosity as a Career

Every morning, I have an interesting task at home. I prepare filtered-coffee and boil milk as soon as I wake up in the morning. Both these processes are supposed to be mundane task, but over the years I have found it to be one of the most intriguing things one can do in kitchen. To make the task engaging, I have been measuring the rate at which half a liter of milk boils and when does it reach the point where it is about to spill over from the container (of course, I do not spill it over, else I will be devoid of my morning coffee…no way). Over many years of this task, I have found that the parameters of milk boiling vary as a function of temperature, humidity, shape of the container, the pressure of the gas supplied in the stove, the content and age of the milk. I have also found some interesting methods to stop the milk spill over even while it is still under boil. In an essence, I start my day with a curious-experiment in the kitchen, and I look forward to it every day.

Radial patterns in a tomato slice on my kitchen slab

Curiosity as life – Tasks like boiling milk, preparing coffee, playing with tooth-paste, running in rain, watching clouds, creating soap bubbles, watching water flow, slicing vegetables (see image), dusting the house, cleaning a window pane, washing shoes and drying an umbrella are common to all of us. If you look at these tasks closely, one can connect them to a lot of interesting science. I have found great joy in doing so, and have turned out be an integral part of my life. An important off-shoot of this way of looking at things is that I hardly get bored. Every trivial thing that I observe has something intriguing, and this has had a profound influence on how I approach my life. Invariably, while exploring my curiosities, I find myself losing the feel for time, and one goes into the state of flow. That is a happy place to park your mind.

Scientists’ dilemma – ‘Impact on society’ is touted as the modern mantra for doing research. A scientist is strongly encouraged, especially by funding agencies, to work on research problems that have relevance to a large community. Even among scientific communities, novel solutions to research problems are often encouraged and are highly valued and rewarded. So, a scientist is always looking for problems that can have greater impact, either conceptually or technically. Influenced by this external push, the priority of what one has to do is always under question. Critically, this puts a scientist in a dilemma: should I work on problems that are curiosity-driven or should I work on problems that have largest impact to the society? This conundrum is especially sharp if one is a scientist whose research requires large infrastructure and financial assistance. Related to this dilemma is the debate of basic vs applied research, and has inspired concepts such as Pasteur’s quadrant. I do research for my living and most of time is spent on it. I and my research group think on the “why and how” of our research, and it is important for us to resolve this dilemma.

Resolving the dilemma, personally – Given that we do laboratory-based experimental research, I have to ensure that we secure research funds to keep it up and running. Concomitantly, I have to cater to my curiosity, without which I will not be able to sustain my interest in the work I do. Over the years, balancing these concerns has influenced the work I do. An important aspect of resolving the above-mentioned dilemma has been to spend long hours on identifying and choosing a research problem that caters to my curiosity and has relevance to the research community. The process of choosing a research problem is not a simple one, but in my opinion, is perhaps the most important step in doing research. After all, the question one defines will eventually guide the answer we can find; hence every minute we spend on it is priceless.

Light and light scattering has been central to all the stuff I do in my research. I am also intrigued by science in everyday life. So, the best possible thing to do was to study light-matter interaction. This inspired me to look for problems that can cater to my interest and a large research community, and may potentially have applications that can impact the society -all of this without having to sacrifice my curiosity. Over the years, this intention has guided me to pursue research at the interface of optical physics and biochemistry; nano-plasmonics, advanced optical instrumentation, and in recent times on plasmon-soft matter interactions. All these areas that I have been working-on are strongly rooted in my curiosities. I have deliberately picked these fields such that I never have to sacrifice on what I like to do.

Parting thoughts – Generally, among research students, there is a concern about their future, and how they can retain their curiosity and pursue their career. Invariably, they are sandwiched between what they like and what the external-world tells them to like. If these two things do not overlap, there is always frustration. For such situations, I have a suggestion: follow your curiosity and be cognizant of the fact that curiosity-driven life not only feeds your brain, but also your stomach. Just by following curiosity, a lot of people including myself, have been able to build a career out of it. What is further encouraging is that there is enough room in the society for our curiosities to develop and flourish, provided we take the effort to connect our curiosity to a relevant research problem out there. This exploration will take time, and we must remain patient until it yields. The onus of connecting our curiosity to external relevance is ours, and we must take the initiative. As the saying goes:

IF IT IS TO BE, IT IS UP TO ME!

My Metaphoric Oxygen

There is no Frigate like a Book
To take us Lands away
Nor any Coursers like a Page
Of prancing Poetry –
This Traverse may the poorest take
Without oppress of Toll –
How frugal is the Chariot
That bears the Human Soul –

BY EMILY DICKINSON

Generally speaking, scientists are natural philosophers: they observe nature, ask questions, hypothesize an answer, test them through experiments and extend this exploration by escaping into the universe of ideas in books and journals. New ideas emerge from this exploration and join the chorus, and the intellectual journey continues. In my own research on light scattering, I have been deeply influenced by ideas of various fellow-explorers. For me, journal papers and books encompass the “metaphorical oxygen” for creativity and knowledge. Below I introduce you to some classic books which keep my research alive.

Absorption and Scattering of Light by Small Particles
- Author(s): Craig F. Bohren and Donald R. Huffman
  - Comments: There are two kinds of authors who write textbooks. One is the ‘boring kind’ and the other is the ‘Bohren kind’. If you want to fall in love with light scattering (and science in general), read books and articles by Craig Bohren. It will not only deeply influence your thinking, but also will show how a textbook can, and should, evolve a subject systematically. This particular classic has some of the most important ideas related to how light behaves when it interacts with matter comparable to the wavelength of light, and forms the bedrock on which a lot of contemporary research, including nanophotonics and plasmonics, is pursued. This book has wit, humour and a touch of poetry jumbled up together as flowing river of knowledge. To give you a spirit of their writings, let me reproduce the first paragraph of their introduction

Bhoren

Light Scatteing by Small Particles
- Author(s): H.C. van de Hulst
  - Comments: The first edition of this book was published in 1957, by the author was a legendary astronomer. This book has a beautiful description of single and multiple-scattering phenomenon, and describes specific situations where they apply. Written with an astrophysical viewpoint, it elegantly combines depth and breadth in a lucid way. This book has perhaps served as inspiration to most of the books written on light scattering.

The scattering of light and other electromagnetic radiation
- Author(s): Milton Kerker
- Comments: Some researchers have remarkable ability to choose problems that have far reaching consequences beyond the next research paper. Milton Kerker was one such legend. His research papers and this book has not only influenced the way physics of light scattering is studied, but has had deep impact on utilization of light scattering in various branches of science and technology. This 600 odd page book is indeed a masterpiece, and in a unique way caters to almost all kinds of researchers who are interested in light scattering.
Dynamic Light Scattering with applications to chemistry, biology and physics
- Author(s): Bruce J. Berne and Robert Pecora
  - Comments: A majority of the matter in biology and chemistry are suspended in a fluid. When an object in a medium undergoes Brownian motion, it influences the way a light beam scatters and traverses through that medium. This book explain the how and why of this fascinating topic. Written by experts in chemical physics, this classic serves as the foundation for light scattering in soft-condensed matter physics.

Molecular Light Scattering and Optical Activity
- Author(s): Laurence Barron
  - Comments: Historically, light scattering by molecules has been studied by legends such as Rayleigh, Raman and many more. Interestingly, all these legends emphasized the connection between polarization of scattered light and structure of matter. In this book, Barron puts together these ideas in a very elegant way, and motivates and develops the phenomenon of optical activity from a molecular physics viewpoint. Given that a majority of biomolecules are chiral in nature, the insight that one obtains by reading this book has direct implication in understanding the structure and dynamics of biomolecules such as amino acids, proteins and DNA.

Scattering, Absorption, and Emission of Light by Small Particles
- Author(s): MI Mishchenko, LD Travis, AA Lacis
  - Comments: Mischchenko is a scientist at NASA, and his books on light scattering have had great influence in aerosol science, radar technology and many more. The T-matrix codes based on this book forms a very important tool across the research community that works on weather prediction and pollution monitoring.

Wave Propagation and Scattering in Random Media (Vol 1 and 2)
- Author(s): Akira Ishimaru
  - Comments: This classic from late 1970s was one of the elaborate attempts to put together wave propagation and scattering in a random media on a rigorous mathematical foundation. This 2 volume book has solutions to various mathematical problems that one encounters in light scattering physics, and makes an important connection to transport theory of light in a medium.

Optical Scattering Measurement and Analysis
- Author(s): John C. Stover
  - Comments: If you are interested in experimental aspect of light scattering, this is one of the best books. It is essentially a field guide, which tells you how to quantitatively make a light scattering measurement, and what aspects to look-out for. This is a very good book for students who want to get a hands-on experience in light scattering.

LASER LIGHT SCATTERING, Basic Principles and Practice
- Author(s): Benjamin Chu
  - Comments: Chu’s book develops the topic of laser light scattering in terms of both experimental aspect and theoretical foundations. Importantly, it connects the topics of light scattering to optical spectroscopy, and shows how one can obtain meaningful information about light-matter interaction.

Mesoscopic Physics of Electrons and Photons
- Author(s): E. Akkermans and G. Montambaux
  - Comments: Quantum mechanical entities such as electrons and photons can be confined in space and time. Depending on the geometry of confinement, very interesting physics such as weak and strong localization can emerge. This book looks at the physics of confined electron and photon from a unified viewpoint. It highlights similarities and difference between the electrons (fermions) and photons (bosons).

The Raman Effect: A Unified Treatment of the Theory of Raman Scattering by Molecules
- Author(s): Derek A. Long
  - Comments: Written by a pioneer in the field, this book till date remains the most rigorous treatment on Raman scattering of light from a theoretical viewpoint. Based on quantum mechanical arguments, this book relies on perturbation theory, and clearly shows the connection between structure of molecules and how they influence the scattered light.

Principles of Surface Enhanced Raman Spectroscopy and Other Plasmonic Effect
- Author(s): Eric C Le Ru and Pablo G. Etchegoin
  - Comments: The most definitive book written on surface enhanced Raman scattering by two physicists whom I greatly admire. This book gives unified treatment of plasmonics and surface enhanced inelastic light scattering, and is written in a style catering to physics audience. The book has a lot of details and explanations, and also serves as excellent introduction to plasmonics and vibrational spectroscopy. Given that the authors themselves are pioneers in single-molecule Raman scattering, their insight into single molecule optics in plasmonic field is fascinating. Unfortunately, Etchegoin succumbed to cancer, and I could never meet him. However his great ideas and thoughts stay on…

Introduction to Wave Scattering, Localization and Mesoscopic Phenomena
- Author(s): Ping Sheng
  - Comments: Random lasing is an emerging topic of research in nanophotonics. The fact that one can have random structures assembled in space and time, and yet achieve spatial and temporal coherence is quite remarkable. This book brings together insights from wave scattering and mesoscopic physics to show how light behaves when confined to small volumes compared to wavelength of light. The insights obtained from this book are heavily used in the literature on random lasers.

Fundamentals of Atmospheric Radiation
- Author(s): Craig F. Bohren and Eugene E. Clothiaux
  - Comments: Bohren weaves his magic…..again. Although the title of this book indicates atmospheric radiation, the way the authors treat the topic of absorption, emission and scattering of light is fascinating. This book gives a broad viewpoint of interaction of light with matter, and shows one can and should treat the subject coherently. The references and problems are very relevant and interesting, and I have found some gems while reading through this text.

Trapping Questions and Evolving Answers

A voice said, Look me in the stars
And tell me truly, men of earth,
If all the soul-and-body scars
Were not too much to pay for birth.

—- “A Question” By Robert Frost

In research, as in life, humble questions can sometimes lead to profound answers. A curious question flying as a passing thought in the mind of a researcher can equally lead to some important discoveries and inventions. Furthermore, what starts as a simple question, evolves into a creature that the questioners themselves would have not envisaged. This evolution of thought in various directions is fascinating to say the least, and history of science is dotted with such examples.

Take for example Arthur Ashkin of Bell Labs, who in late 1960s, asked the following question:

“is it possible to observe significant motion of small particles using the forces of radiation pressure from laser light?”

Note- at that point of time, lasers were still a relatively new invention, and people were looking for an application. In that context, it was indeed an interesting question to ask about the effect of laser beam on a small particle which may be immersed in fluid or in vacuum. After all, radiation pressure should have some effect on the motion of particles, as evidenced in the case of comet tails.

With this question, Ashkin embarked on a journey that conceptually and literally pushed and revolutionized a large part of our science and technology based on lasers. Ashkin’s question led to the realization of laser-based optical trap of microscopic objects, which further evolved into a major experimental tool not only in physics but also in biology and chemistry.

Below figure shows the conceptual schematic of Ashkin’s experiment, in which he introduced two counter-propagating laser beam which created an optical potential to stably trap an object in space and time. The physics of optical trapping itself in intriguing, in which, the compelling battle between forces due to in-line pushing and orthogonal pulling will be eventually won by the pulling component. A stable energy minimum is achieved at the center of the focused laser beam, in which the object of interest happily resides. Of course, parameters such as refractive index of the object and the medium play a critical role, so does the alignment of laser beam and its wavelength.

trap Ash — Optical schematic of the first optical trap created by Arthur Ashkin. Adapted from the original paper [1]*.

There are two important aspects to Ashkin’s work. One is that he pursued on a simple question that lead to an important observation, which has had far researching consequences not only in physics but also in biology and allied research areas, and the second point is that a few people, in his own lab felt that the discovery was not important. In the first chapter of his book, Ashkin describes a very interesting situation after he had performed this seminal work:

It may be interesting and instructive to recall the initial reactions of other scientists to paper [1]*, which described the earliest trapping work. At Bell Labs., before a manuscript could be sent out to a journal it had to undergo an internal review to make sure it would not tarnish the laboratory’s excellent reputation in research. Since paper [1]*was intended for Physical Review Letters, it was sent to the theoretical physics department for comment. The Bell Labs, internal reviewer made only four points: (i) there was no new physics here, (ii) the reviewer could not actually find anything wrong with the work (this is a reminiscent of the famous Pauli insult, when he commented on some work he thought worthless that “it is not even wrong!”), (iii) the work could probably be published somewhere, and (iv) but not in Phys. Rev. Lett.This four-point internal referee report from the theoretical group greatly distressed me, and so I went to my boss, Rudi Kompfner, inventor of the traveling wave tube, whom I greatly admired. Rudi, a man usually slow to anger, simply said, “Hell, just send it in!” As it turned out, I had no problem whatever with the Physical Review Letters reviewers. In 1999, paper [1]* had the honor of being selected as one of the 23 seminal papers on atomic physics reprinted in the compilation, “The Physical Review — The First Hundred Years”, edited by Henry Stroke, American Institute of Physics Press and Springer Verlag (1999) on the occasion of the centennial of the American Physical Society.

There are at least two important lessons in this story: a) not always one can instantaneously judge the importance of a research work and b) the notion of “new physics” depends on how you look at a topic and judge its implication. To see how a new result can connect to something else requires a kind of broad view of science well beyond the boundaries of the “known unknowns”.

Going further, Ashkin did not stop his train of questions. He writes that he was intrigued by the observations which further motivated him to explore on the following topics:

Could traps be observed for macroscopic particles in other media such as air or even in a vacuum? Could optical manipulation be used as a practical tool for studying light scattering, for example, and other properties of small macroscopic particles?

Evolution of Ideas

After some resistance, slowly the physics community started taking notice of Ashkin’s experiments, and paid more attention towards the simple yet powerful methods he was developing. What followed was indeed a revolution. The methods he developed immediately caught the attention of two very diverse research communities – one was of atomic physicists and other one was of biologists. Whereas the former were interested in trapping and cooling atoms, the later were in desperate search for non-invasive optical tools that could trap and manipulate cellular and sub-cellular objects. Optical trapping indeed catered enormously towards these research efforts. It not only led to “new and interesting physics”, but also some wonderful experiments in soft-matter and biological sciences. In order to give you a gist of the way Ashkin’s work evolved, below I give a table of interesting research results. As you will see, the papers themselves discuss topics and problems that were not envisaged by Ashkin, but the influence of his ideas percolated deep and wide.

Year	Link to the relevant papers and my comments
1982	Electromagnetic mirrors for neutral atomsThis paper theoretically proposed use of evanescent optical fields at dielectric-vaccum interface to reflect neutral atoms. The concept of radiation pressure at an interface was emphasized.
1986	· Three-dimensional viscous confinement and cooling of atoms by resonance radiation pressure· Experimental observation of optically trapped atoms These were the foundational experiments on laser cooling and trapping of atoms, which went on to win the 1997 Nobel Prize in physics. Note that Ashkin missed out on the prize!
1989	Optical Binding This introduced a fascinating concept of binding microscopic objects with long range optical forces facilitated by electromagnetic fields. This topic is still of great interest, and still inspires a variety of experiments.
1992	Movement of micrometer-sized particles in the evanescent field of a laser beam This paper was a pioneering contribution towards movement of particles in fluids using an evanescent wave of laser beam.
1993	Direct observation of kinesin stepping by optical trapping interferometry The abstract of this paper is worth a read and tells a compelling story :“Do biological motors move with regular steps? To address this question, we constructed instrumentation with the spatial and temporal sensitivity to resolve movement on a molecular scale. We deposited silica beads carrying single molecules of the motor protein kinesin on microtubules using optical tweezers and analysed their motion under controlled loads by interferometry. We find that kinesin moves with 8-nm steps.”
1996	Optical vortex trapping of particles This was one of the first experiments to use vortex beams to trap objects. In conclusion of the paper, the authors envisage trapping application based on holograms, which were created soon after the proposal.
1997	Theory of nanometric tweezerA first significant jump towards extrapolating optical trapping to sub-wavelength scales. The idea of utilizing a metal nano-tip to trap dielectric objects was proposed. This paper laid an excellent foundation for optical manipulation at nanometer scale.
1998	Optical tweezer arrays and optical substrates created with diffractive optics This literally added new dimensions to optical trapping, where a diffractive optical element, a static hologram in this case, was introduced in the optical scheme. This laid the foundation towards parallel trapping on conventional set-up, and has turned out to be extremely useful for applications in soft-matter physics and biological applications.
2001	Force of surface plasmon-coupled evanescent fields on Mie particles This theoretical paper compares how evanescently-excited surface plasmon polaritons at metal-dielectric interface can exert more force on Mie particle compared to a dielectric-dielectric interface, thus creating a platform for film-based plasmonic manipulation of micro-objects.
2006	Surface Plasmon Radiation Forces This was the first report that experimentally showed how surface plasmon from a metal interface exerted about 40 times more force on a micron sized particles compared to a dielectric interface. Importantly, this paper measure the trapping potential depths created by surface plasmon on a metal-film.
2007	Parallel and selective trapping in a patterned plasmonic landscape This was perhaps THE BREAKTHROUGH experiment in plasmonic trapping, that showed how gold nano-disc could create parallel traps of micron scale object at significantly lower power compared to optical trapping. A majority of plasmon trapping experiments nowadays derive their inspiration from this paper
2009	Self-induced back-action optical trapping of dielectric nanoparticles This experimental paper is one of the first reports which harness the feedback from the trapped 50 nm object to improve the performance of the trap. This significantly reduces the power of laser one needs to use for trapping experiments and represents truly a nanometric optical trap.
2010	Laser Printing Single Gold NanoparticlesOptical trapping forces are harnessed to printing individual gold nanoparticles on glass substrates. This has opened up new opportunities to directly fabricate nanostructure from colloidal phase onto a surface of interest.
2012	Subkelvin Parametric Feedback Cooling of a Laser-Trapped Nanoparticle To quote the authors “Using a single laser beam for both trapping and cooling we demonstrate a temperature compression ratio of four orders of magnitude”. This opens a new avenue to perform optical tests of quantum mechanics using isolated nanoparticles.
2014	Plasmofluidic Single-Molecule Surface Enhanced Raman Scattering from Dynamic Assembly of Plasmonic NanoparticlesThis is an experiment report from my group where we showed that one could not only create a large scale plasmonic trap of multiple nanoparticles, but also one can utilize it to perform single-molecule spectroscopy.
2016	Direct Measurement of Photon Recoil from a Levitated Nanoparticle Another experimental breakthrough where the photon recoil from a single nanoparticle is measured.
2018	Opto-thermoelectric nanotweezers This experimental paper shows how optical, thermo-plasmonic and electric fields can be combined to trap and manipulate nano-object in fluids.

To conclude I will again quote Ashkin, who makes an important observation in an editorial he wrote on the occasion of commemorating 50 years after the discovery of laser:

As we look to the future, what can we anticipate? Certainly much more of the present hot fields such as: single atom studies; properties and behavior of single biological molecules such as mechanoenzymes and nucleic acids; mechanical properties of single molecules and tissue; studies of particle arrays; and particle separation schemes. Of course, we cannot anticipate serendipitous discoveries. We can only hope to recognize them when they occur.

After all, one question leads to another….and the rest is evolution…you see!

Science Behind the Science Day

Every year on 28th February India celebrates National Science Day. Although, there is quite a bit of enthusiasm in this celebration, especially in academic environment including schools, universities and research institutes, the general public does not pay too much attention to it. Partly because symbolic days and symbolism are just that: symbols – superficial representation of something bigger. They do not encompass the complete picture, and they are too many in number.

Raman + Krishnan Effect : So, why does India celebrate National Science day on 28th February ? Well, on this day, way back in 1928, there was an experimental observation performed which turned out to be an important discovery in science. The main players in this discovery were two scientists from India: C.V. Raman and K.S. Krishnan. These men, after a long, sustained effort and with limited experimental resources, discovered a new type of secondary radiation, and the effect what is now known as Raman effect.

Stokes and anti-Stokes: The experiment that they were performing was to look at monochromatic (single colour) light scattering from molecules in a liquid. What Raman and Krishnan found was that scattered light had three components in terms of energy: the first and the dominant part of the scattered light had the same energy as the incident light, the second most dominant part of the scattered light had its energy lower than the incident light, and the third component, which was the weakest, had its energy greater than the incident light. These three components are called Rayleigh, Stokes and anti-Stokes light, respectively. The Stokes and the anti-Stokes components of the scattered light, together encompass the inelastic scattering in the process, and formally represent Raman scattering of light from molecules. What is remarkable on the part of Raman and Krishnan is that they experimentally observed these feeble, inelastic scattering components with ingenious experimental design. The story of this discovery is beautifully captured in the book The Raman and his effect by G. Venkatraman (who has also written a biography of C.V. Raman). In this book (page 46), we get a glimpse of the lab notes of K.S. Krishnan during this historical phase of experimental observation, which is reproduced below:

February 17, Friday
Prof, confirmed the polarisation of fluorescence in pentan vapour.
I am having some trouble with the left eye. Prof, has promised to
make all the observations himself for some time to come.
February 27, Monday
Religious ceremony in the house. Did not go to the Association.
February 28, Tuesday
Went to the Association only in the afternoon. Prof, was there and we
proceeded to examine the influence of the incident wavelength on the
phenomenon. Used the usual blue-violet filter coupled with uranium
glass, the range of wavelengths transmitted by the combination being
much narrower than that transmitted by the blue-violet filter alone.
On examining the track with a direct vision spectroscope, we found
to our great surprise that the modified scattering was separated from
the scattering corresponding to the incident light by a dark region

Note that it is this February 28, that we celebrate as National Science Day.

Mud + gold – I see an important lesson from this reading – that lab note books are an excellent window to the world of observations that a researcher experiences. Most of the time what is written in a lab notebook is routine stuff, but once in a while there is something important that pops out from it. The analogy is similar to digging for gold in the soil. Most of the time it is mud what you get, but once in a while you extract the precious metal. But without the effort of digging, one will never be able to extract the gold, and a lab notebook is the place where you record your efforts. So, what comes out as “success” is a small part of this greater effort. It is a tiny bit of a greater whole, and worth every bit.

new type of — Title and a part of the abstract of the paper reporting Raman effect (reference: *Nature* **volume121**, pages501–502 (31 March 1928))

Coming back to Raman and Krishnan, they published (see above) their experimental observation which caught the attention of scientific community around the world (‘western’ world to be more precise). And in 1930, Raman went on to win the Nobel prize in physics, and the rest is history. I need to emphasize that this discovery was made purely to address a quantum mechanical effect in optical regime. Specifically, the researchers were addressing an optical analog of the Compton effect, and they had no immediate applications in their mind. However, in the current age, Raman scattering spectroscopy, has bloomed into one of the most important scientific concepts, and a vital tool in science and technology, including applications in clinical bio-medicine and homeland security.

Prof. Wolfgang Kiefer showing his Raman scattering instrument which he has set-up in the basement of his house

The celebration: It has been 90 years since this important discovery, and to celebrate the discovery, there was a conference at IISc, Bangalore. Some of my students and I were part of it. In there, various researchers across the world including many from India, discussed about Raman scattering and its implication over the past 90 years. One of the highlights of the conference was a lecture by Prof. Wolfgang Kiefer (given on 28th February, National Science Day), who is a legend in the community of Raman scattering. Prof. Kiefer has been working on Raman scattering for more than 50 years. Now, he is retired, but amazingly, maintains a lab in the basement of his house, in which he has set-up Raman scattering experiments (see picture above), and pursues his curiosity with child-like enthusiasm. He gave an overview of his work done with his illustrious students from the past, and beautifully blended science, humor and humanity in a single talk. To listen to him was a pleasure and inspiration, and I will remember this for ever. On a personal note, I was actually celebrating the science day without realizing it !

Why I do Science ?

December is a month when you meet a lot of people, especially if you are an academic on a semester break. Invariably one is traveling on conference, or is on an outing with family during this month. Many a times, you meet new people, especially if you are traveling to new places. A general question people ask is: what do you do for a living? This is easy to answer (I am a scientist/professor working at IISER….blah..blah..), but once in a while somebody (generally a child) enquires : why do you do science? As always, the ‘why’ questions are not trivial, and needs a bit of thinking.

When I was asked about this why question recently, my ‘short answer’ was: because I like asking questions. Later, I thought about this question, and below is my ‘not-so-short’ answer:

A playground to wonder – The main driving factor of why I chose science is that I love asking questions and wonder about it. I found that, science and scientific research gives me a metaphorical playground to wonder about questions I have in my mind. In fact, I earn my living doing this! For me this is of fundamental importance to my living: the freedom to ask questions. In an essence, doing research is all about asking questions and trying to find out an answer. The answer you may find need not be complete (or correct), so you will have to again ask another question to verify that answer, and this process continues over many iterations, until you have viewed the answer from various different perspectives, and have come to a satisfactory answer. Remember that there is always room to ask more questions, so the process never ends. But you are allowed to pause or switch to a new question, after you have asked many questions. As you may observe, what drives you forward is the question, and this process of “Q & A” is perpetual in research, and I absolutely love it.
Mind + Hands – Being an experimentalist, I get a great intellectual kick by asking questions and creating new things in the lab. I can see how ideas in our minds can (or cannot) be realized by working with our hands, and this is a tremendously exhilarating, enabling and humbling process. Whenever you build a new instrument (an optical instrument in our case), it is no more just an equipment, but it is a piece of art built with labor of love. In a strange way, one also forms a bond with the instrument you build. That feeling is unique to the creator, and many of my students have tasted this high.
Visiting the past – I love history, especially history of science. It gives me an opportunity to intellectually explore the past in detail. In order to formulate a question, I need to understand the history behind the question. I need to know what other people have thought about the question, and how they have approached and addressed it. Science, after all, is built on ideas from the past and present, so knowing the history is vital. In a fast-paced world, this ability to explore the past is rare, but in research it is prevalent.
Science as human expression – Human being is a social animal. They interact, talk, exchange ideas and learn. To me science is a form of human expression. It adds two important dimensions to society. The obvious one is the utilitarian aspect. Science benefits mankind through its application. In fact everything around our materialistic world is thanks to science and its off-shoot – technology. The non-obvious dimension that science inculcates is the way of thinking and looking at the world. Science facilitates a metaphorical spectacle to view the universe. As Feynman describes beautifully in this video, science helps you to appreciate the world you live in, just as art does.
Test of the self – A scientist needs all the ability that any top professional needs: endurance, concentration, time- and people-management skills, and deeper understanding of ethics, rights and duties. Doing scientific research, especially as a profession, needs coordinating and interfacing with people. And wherever there are people, there are infinite parameters to deal with. Doing science is not just about being in the lab or at your desk/board; it is an activity which generally happens in sync with a community. Interacting with such a community, interestingly, will bring the individual in you. It will help you identify yourself and differentiate from others, in a positive way. I have realized and understood myself more and better by interacting with others than by keeping thoughts to myself. This process of self-realization by communicating with the outside world has been a great learning experience. You expose yourself not only to new science, but also to new ways of living and thinking, especially when you travel and meet new people from different cultures. This exposure to the unknown has added a new dimension to my views and has enriched my life. After all, diversity has its use.

So, these are the main reasons why I do science. Doing what we do as profession is an extremely personal thing. Not everybody gets an opportunity to do what they want, but if you get it, you better nurture it with love and passion. As the English poet Alexander Pope said “On life’s vast ocean diversely we sail. Reasons the card, but passion the gale.”

So add more gale to your life….and have a WONDERFUL year ahead !

Born and Wolf

BORN AGAIN: Today I opened the google webpage and to my surprise found the doodle (picture above) celebrating birthday of Max Born. He was not only a great physicist who contributed immensely to quantum mechanics and other branches of physics (including optics), but also a mentor to many great physicists including Fermi, Heisenberg, Pauli, Wigner, Teller, Emil Wolf and many more.

Every student who has studied physics, is aware of quantum mechanical wavefunction (ψ). Given a quantum system and its environment (electron in an atom, for example), wavefunction is a fundamental quantity that one can compute, and forms the basis to understand the system in greater detail. When quantum mechanics was evolving in early 1900s, the question of how to physically interpret the meaning of wavefunction was at the forefront. It was Max Born who gave the statistical interpretation for the wavefunction, which later fetched him a Nobel prize in 1954.

Born identified the importance of interpretation of the wavefunction, and its connect to the realistic, observable parameter. To quote Born from his Nobel lecture :

“The problem was this: an harmonic oscillation not only has a frequency,
but also an intensity. For each transition in the array there must be
a corresponding intensity. The question is how to find this through the
considerations of correspondence? “

This quest set forth an intense programme in physics and motivated people like Heisenberg, Schrodinger, Bohr, and Einstein to find an answer. Interestingly, Born’s work was heavily inspired by Einstein’s work. To quote Born from his Nobel lecture:

“But the decisive step was again taken by Einstein who, by a fresh
derivation of Planck’s radiation formula, made it transparently clear that the
classical concept of intensity of radiation must be replaced by the statistical
concept of transition probability.”

Further, he adds

“Again an idea of Einstein’s gave me the lead. He had tried to make the duality of particles light quanta or photons – and waves comprehensible by interpreting the square of the optical wave amplitudes as probability density for the occurrence of photons. This concept could at once be carried over to the ψ-function: |ψ|^2 ought to represent the probability density for electrons (or other particles).”

Also, read an interesting commentary by A Pias on “Max Born and Statistical Interpretation of Quantum Mechanics“.

Reading Born’s Nobel lecture, two things struck me : first was that science is never done in isolation. Every single idea is inspired by another idea. Second, physical optics has a major influence on interpretation of quantum mechanics. Max Born was no stranger to optics. In fact, he was one of the pioneers of classical optics, and I am not surprised that he could make some vital connections between physical optics and quantum mechanics.

BW book — My personal copy…..standing tall and heavy :)

THE BOOK: This brings me to the most famous book written in optics(see picture above) by none other than Max Born and Emil Wolf (Emil Wolf was the last research assistant of Max Born, and a well know optical physicist) The book is titled “Principles of Optics”, but in optics community we call it “Born and Wolf”. The first edition of this book appeared in 1959, and has never gone out of print. Currently, it is in its 7th edition and is 951 pages thick !

As described in the preface (first edition of Born and Wolf), several people urged Born to translate his 1933 book: “Optik” from german to english. By 1950s, optics had evolved and had made inroads into atomic physics, molecular spectroscopy, solid-state physics and various other branches of science and technology. So, they had to write the book from scratch taking new ideas into consideration.

“Born and Wolf” explains optical phenomenon through the eyes of Maxwell’s theory, and has become the foundation on which various aspects of classical optics can be studied in a mathematically rigorous fashion. In fact, it also lays foundation to various quantum optical phenomenon including coherence and correlation functions, on which Emil Wolf’s contribution has been immense.

For me, chapter 13 on “Scattering from homogeneous media” is the highlight of this book. It starts with elements of scalar theory of scattering by expaining the first-order Born approximation followed by discussion on scattering from periodic potential. The best part is the discussion on multiple scattering, which in a sense lays the foundation to study various important optical phenomenon including diffraction tomography and optical cross-section theorem (or more famously known as Optical theorem). Also, the 13th chapter has a very interesting discussion on concept of far-field and its connection to scattering of electromagnetic waves.

Actually, the book is very well known for its treatment on diffraction theory and image formation. It gives a very strong footing to attack problems in imaging, aberration and inteferometry using Maxwell’s equation and related boundary condition. It also, highlights optics of metals, which has now transformed and evolved into a sub-field of optics and photonics – plasmonics.

Origins of the book: The writing of this book has a historical context. Emil Wolf was a research assistant (post-doc) of Max Born and joined him after his Ph.D. He recollects his experiences with Born and about writing this book in an interesting article. Below is an interesting quote:

“Through Gabor I learned in 1950 that Born was thinking of preparing a
new book on optics, somewhat along the lines of his earlier German book
Optik, published in 1933, but modernized to include accounts of the more
important developments that had taken place in the nearly 20 years that
had gone by since then. At that time Born was the Tait Professor of Natural
Philosophy at the University of Edinburgh, a post he had held since 1936,
and in 1950 he was 67 years old, close to his retirement. He wanted to find
some scientists who specialized in modern optics and who would be willing
to collaborate with him in this project. Born approached Gabor for advice,
and at first it was planned that the book would be written jointly by him,
Gabor, and H. H. Hopkins. The book was to include a few contributed
sections on some specialized topics, and Gabor invited me to write a section
on diffraction theory of aberrations, a topic I was particularly interested in
at that time. Later it turned out that Hopkins felt he could not devote
adequate time to the project, and in October of 1950, Gabor, with Born’s
agreement, wrote to Linfoot and me asking if either of us, or both, would
be willing to take Hopkins’ place. After some lengthy negotiations it was
agreed that Born, Gabor, and I would co-author the book.”

Wolf writes about Born and his working style:

“In spite of his advanced age Born was very active and, as throughout all
his adult life, a prolific writer. He had a definite work routine. After coming
to his office he would dictate to his secretary answers to the letters that
arrived in large numbers almost daily. Afterward he would go to the adjacent
room where all his collaborators were seated around a large U-shaped
table. He would start at one end of it, stop opposite each person in turn,
and ask the same question: “What have you done since yesterday?” After
listening to the answer he would discuss the particular research activity and
make suggestions. Not everyone, however, was happy with this procedure.
I remember a physicist in this group who became visibly nervous each day
as Born approached to ask his usual question, and one day he told me that
he found the strain too much and that he would leave as soon as he could
find another position. He indeed did 80 a few months later. At first I too
was not entirely comfortable with Born’s question, since obviously when one
is doing research and writing there are sometimes periods of low productivity.
One day when Born stood opposite me at the U-shaped table and asked,
“Wolf, what have you done since yesterday?” I said simply, “Nothing!” Born
seemed a bit startled, but he did not complain and just moved on to the next
person, asking the same kind of question again.”

Wolf also gives an account of why Gabor pulled-out, and how Wolf had to play an unexpected, but vital role in writing this book:

“…..Gabor soon found it difficult to devote the necessary time to the project, and it was mutually agreed that he would not be a co-author after all, but would just
contribute a section on electron optics. So in the end it became my task to
do most of the actual writing. Fortunately I was rather young then, and so
I had the energy needed for what turned out to be a very large project. I
was in fact 40 years younger than Born. This large age gap is undoubtedIy
responsible for a question I am sometimes asked, whether I am a son of the
Emil Wolf who co-authored Principles of Optics with Max Born!”

Wolf also praises Born’s open-mindness to various branch of physics:

“Optics in those days-remember we are talking about optics in pre-laser
days-was not a subject that most physicists would consider exciting; in fact,
relatively little advanced optics was taught at universities in those days. The fashion then was nuclear physics, particle physics, high energy physics, and
solid state physics. Born was quite different in this respect from most of his
colleagues. To him all physics was important, and rather than distinguish
between “fashionable” and “unfashionable” physics he would only distinguish
between good and bad physics research.”

Emil Wolf is now 95 years old, and is still a very active researcher. His recent paper was in 2016 on partially coherent sources and their scattering from a crystal. Wolf’s books are classics in optics, and continues to raise probing questions and important connections in sub-branches of optics.

In an essence, great science books are written with love and passion to communicate the excitement of science. Born and Wolf certainly does that, and continues to inspire us to learn optics from the masters themselves.

To conclude, let me quote Born himself from his Nobel banquet speech:

“The work for which the Nobel Prize has been awarded to me is of a kind which has no immediate effect on human life and activity, but rather on human thinking. But indirectly it had a considerable influence not only in physics but in other fields of human endeavour.

This transformation of thinking in which I have taken part is however a real child of science, not of philosophy: it was not the result of speculation, but forced upon us by the observed properties of Nature.”

Max Born and Emil Wolf, your work and your books have transformed our thinking, and the way we see light and matter. Thank You !

Biological Lasers – Alive and Emitting

Biolaser1

Laser, laser burning bright,

In the lab, through the night

What immortal ideas

Could frame thy emission process?

–modified from William Blake’s ‘The Tyger’

Light Amplification by Stimulated Emission of Radiation or LASER is a light source which is ubiquitous in the world around us. They are extensively used in scientific laboratories to study phenomenon spanning various sizes: from astronomy to sub-atomic particles. A laser has three vital characteristics: they are a monochromatic, coherent and highly directional in nature, which make them unique emitters of light. Unlike conventional light sources, such as tube-lights, lasers carry a relatively large amount of energy in a confined volume, and hence can propagate over a longer distance and strongly interact with matter. These properties of lasers have not only made them a fascinating topic of fundamental research, but also play a critical role in various applications.

Vital Quest: Almost all the lasers that have been produced are made of abiotic matter, that is, matter that does not have life. An interesting question to ask is: can we create a laser out of a living system, such as biological cell?

The answer is YES. In 2011, two optical physicists, Malte C. Gather and Seok Hyun Yun, then at Harvard Medical School, came up with an interesting experiment. They transfected green florescent proteins inside a human embryonic kidney cell, and placed the cell between two high-quality mirrors, and optically pumped the cells with blue pulsed light (see figure 1(a)). Interestingly, this experiment resulted in lasing action at around 513nm wavelength from various parts of the biological cell (see figure 1(b)), and thus a biological laser was realized. The green laser light originated from the transfected green florescent protein inside the cell, which essentially acted as a gain media inside an optical cavity. The emitted light had rich spatial structure as evidenced in figure 1(b), and this structure depended on the local distribution of the green fluorescent proteins. This is the first report, to my knowledge, where a living system, such as a biological cell, has emitted laser light, and has literally created tremendous ‘excitement’. So one may ask, what the uses of such a system are.

Biolaser2 — **Figure 1**: (a) Experimental schematic to realize biological laser. The system consists of a human-embryonic-kidney-cell transfected with green florescent protein. This cell was placed between two high quality mirrors which are separated by a distance ‘d’ (representing an optical cavity). The cell was optically pumped by blue pulsed light (465 nm wavelength). This resulted in laser emission from the cell. (b) Optical image of a lasing human embryonic kidney cell. The green light (wavelength around 513nm) emanating from the cell is the laser light. The scale bar is 5 μm and the colour bar represents increasing intensity from dark to light shade. Figures reproduced with permission from *Nature Photonics, 5, 406-410 (2011).*

Bright Future: The research on biological lasers is in its infancy. There are many interesting prospects of such lasers, and I outline a few of them:

They can act as a localized source of light and heat, which may further drive certain mechanical, thermal and chemical reactions in organelles and compartments of a cell.
If placed in an appropriate location inside living systems, biological laser can be harnessed as light sources in biomedical applications, where the reach of certain surgical instruments is constrained.
The emanating light is due to stimulated emission, which has narrow spectral width. In contrast to spontaneous emission, which is the process behind conventional light emitters, lasers have very narrow spectral widths, and hence can be utilized for spectral multiplexing and discrimination of species inside a cell.

Voyage ahead: There are still many unexplored aspects of a biological laser. Below I mention three of them from different viewpoints:

From optical physics viewpoint, a biological laser can be treated as an optical cavity with a randomly-scattering gain media. It would be interesting to explore the localization, propagation and directional emission of light in such a randomized medium inside a living system.
From chemistry viewpoint, it is interesting to ask if one can design and synthesize bio-compatible macromolecules or nano-materials which can be placed inside a cell such that it leads to efficient self-lasing without external stimuli.
From biology viewpoint, it is vital to know what will be the fate of a cell if it keeps emitting laser light. Specifically, it would be interesting to know how cells can adapt to an in-built laser source. Furthermore, if a cell with biological laser splits into to two, will it carryover its ability of lasing to the next generation?

As with all interesting inventions and discoveries, biological lasers have opened many interesting questions to be explored. It has room for contribution from various branches of science and technology, and may open new avenues by bringing together biology and laser photonics. Let me conclude by quoting Feynman:

“…..A biological system can be exceedingly small. Many of the cells are very tiny, but they are very active; they manufacture various substances; they walk around; they wiggle; and they do all kinds of marvelous things—all on a very small scale. Also, they store information…..”

Well Mr. Feynman, now they can even emit laser light!

Polluted air…..why do we care ?

If you are an Indian researcher, you cannot escape a visit to Delhi. For the last few years, I have been visiting Delhi for various research related reasons: conference, grant meeting etc. A few days ago I had an opportunity to visit Delhi for a half-a-day meeting.

For me, Delhi embodies a rich feeling of delicious north Indian (street) food, extreme temperatures (by Indian standards), loud taxi music, an assorted flavor of Hindi dialects, and of course, national politics. Of late, Delhi also has gained a lot of attention in another matter: pollution. In winters, for several years now, smog (smoke + fog) has been a major problem, and has drastically perturbed the lives of Delhi citizens. This problem is not confined to Delhi. Various parts of India are not doing great either.

Anyway, as soon I landed in Delhi, it was foggy (see picture), and the visibility was poor. Clearly, there was something in the air, and it was not pleasant. I wondered about all the kids who travel to school in such an air, and the possible effects on their health. There were several questions running through my mind: Why is the polluted air the way it is ? How does one quantify pollution? What are the effective methods to detect pollution, and how can it be contained effectively? I knew some scientific aspects of air pollution, but I was curious about how at all air quality was measured and quantified. Below are some facts related to pollution and some interesting connection to light scattering.

There are several reasons for air pollution. In India, some of the major reasons include crop and biomass burning, emission from automobiles and industries, dust etc. There are mainly 8 kinds of pollutants: PM ₁₀ , PM _2.5 , NO ₂ , SO ₂ , CO, O ₃ , NH ₃ , and Pb. A majority of the problems are caused by the so-called particulate matter. These tiny objects which can cause severe harm to human beings can be mainly classified as PM10 and PM2.5, where PM stands for particulate matter and the numbers 10 and 2.5 represents the size of such particles in microns. To give you a comparison, the width of our hair is approximately 100 microns in thickness. So imagine a particle which is thinner than your hair entering your respiratory system. This inhalation causes severe trouble to your lungs and the worst part is that it can cause irreversible damage to the inner walls of your respiratory tracts. Even more disturbing fact is that smaller the size of the particle deeper is the penetration in to our system, and greater harm it does to human well-being.

So, how to detect these small particulate matter? There are several ways to detect these tiny objects of which I found two methods to be interesting and effective.

First one is based on light scattering. Generally, the instrument used to monitor air quality using light scattering is called as nephlometer (In Greek nephos means cloud). This is a powerful and compact instrument that can continuously detect and monitor density of particulate matter. The measurement is based on Mie scattering (named after Gustav Mie, more about him in future), where the size of the scatterer is generally comparable to the wavelength of light. It assumes that the scattering particles are spherical in nature and isotropic in composition. It works on the basic principle that when you shine light through smog (at the ground level), the intensity of the scattered light carries characteristic signature of the size of the particle and its concentration. More specifically, the intensity of the scattered light depends on two important ratios. One is the ratio of particle size to wavelength of light and second is the ratio of refractive index of the particle to its surrounding medium. By calibrating the instrument for known particles and concentrations, the unknown size and concentration of the pollutant can be determined. (If you are interested to learn more, see this old research paper). As mentioned earlier, the measurement assumes the scatterer to be spherical and isotropic, which is not the usual case in the air. So corrections due to variation in shape and compositions have to be taken into consideration in this measurement. However, one of the major advantages of this measurement is that it is quick and portable, and hence a lot of air quality measurements are based on these instruments.

Alternatively, if one needs very accurate measurement of particle size, the instrument to use is Tapered Element oscillating microbalance (TEOM). In this a tiny piece of tapered glass acts like a tuning fork. This tuning fork vibrates at a specific frequency which can be measured with reasonable accuracy. As one may guess, if something is moving, the speed of movement can be affected by adding weight on the moving object. In this case, the vibrating piece changes its frequency as soon as a small particle is in contact with it. The difference in the frequency is now related to the mass of the particle. Thus by using simple physics, one can obtain a powerful instrument to monitor air quality. Apart from the above-mentioned methods, there are various approaches to monitor air-pollution. Each of them have pros and cons, and are utilized depending on the situation.

Coming back to my Delhi trip, I finished my work, and headed towards the airport in a taxi. I casually asked the driver whether he was worried about the pollution in his city. He did mention that it was a concern, but after a brief pause he grinned and said – “odd-even phir sae shooru ho ra hain, business badega” (odd-even is starting gain, business will go up (note: odd-even was eventually stalled this time)). I grinned back at the driver, and remembered a quote of Charles Kettering : “The only difference between a problem and a solution is that people understand the solution”.

Colourful Sky in Leonardo’s Eye

There are very few people in human history who have combined arts and science like Leonardo da Vinici. He was a polymath: a great painter, inventor, sculptor, scientist and as you will see – a keen observer of nature, and many more. One of the great aspects of Leonardo is that he recorded his observation as texts, which gives us a deep and direct insight into his thinking.

All of us have been captured by the beauty of sun lit sky. It has made us gaze and wonder about its colour. Of course, it has inspired a countless number of artist and scientist to ask the question : what is the origin of colours of a sun-lit sky ? Leonardo himself was fascinated by this question, and led him to view this question both as a painter and as a scientist. Thanks to the great work of J.P. Richter, who has translated the Literary works of Lenardo da Vinici into english (available free online), we obtain a direct peek into the mind of Leonardo which is an everlasting treasure trove: more you dig more you get.

Leo2

The title page of the translated book

In the collected works, what has caught the attention of scientists is a chapter titled “Aerial Perspective”. In there, Leonardo is trying to converse with his fellow painters on how to create perspective in paintings. While doing so, he makes some vital observation and proposes hypotheses, and further discusses about some experiments to test them. Leonardo was a keen observer. His approach to art was heavily influenced by an analytical way of looking at the problem at hand. In his writings, he appeals to painters to pay close attention to angles and perspectives in the geometry. In order to attain precision he gives elaborate explanation based on his observation. Below is an example where he explains how to represent the atmosphere in paintings.

“Why the atmosphere must be represented as paler towards the lower portion? Because the atmosphere is dense near the earth, and the higher it is the rarer it becomes. When the sun is in the East if you look towards the West and a little way to the South and North, you will see that this dense atmosphere receives more light from the sun than the rarer; because the rays meet with greater resistance.”

It is remarkable how his efforts to create a painting inspired him to go deeper and hypothesize a physical phenomenon. Below sentences reveals a connection he makes between colour of the sky and the presence of “insensible atoms”.

“ I say that the blueness we see in the atmosphere is not intrinsic colour, but is caused by warm vapour evaporated in minute and insensible atoms on which the solar rays fall, rendering them luminous against the infinite darkness of the fiery sphere which lies beyond and includes it”

Although, now we know that light scattering from molecules (not atoms) as the reason for colourful sky, we need to really appreciate Leonardo’s quantum leap of thought. Remember, his texts are dated around late 1400s or early 1500 AD, where the presence of atoms and molecules were not yet verified. As a person with scientific aptitude, Leonardo not only hypothesized, but also tested them with experiments. Below he refers to a beautiful experiment with smoke and the perception of colour arising due to the background.

“Again as an illustration of the colour of the atmosphere I will mention the smoke of old and dry wood, which, as it comes out of chimney, appears to turn very blue, when seen between the eye and the dark distance. But as it rises, and comes between the eye and the bright atmosphere, it at once shows of an ashy grey colour; and this happens because it no longer has darkness beyond it, but this bright and luminous space.”

To me this is nothing but a first rate example of looking at nature through a scientific eye, and adapting this view as means to a certain end. It is a tribute to Leonardo who paid such meticulous attention to details, and attempted to explain an unexplained physical phenomenon – all in the name of getting a painting right ! This is also a wonderful example of how aesthetics and science combined in the mind of Leonardo, which further led to some breathtaking work. After all, science and arts are two aspects of human expression, and Leonardo combined them effectively.

There is another lesson we can learn from such endeavours: observations play a key role. Sometimes, when a student is working in a laboratory, she or he may wonder why one should keep records of ones observations. Well, to them I say – look at Leonardo, he took an important step to write down his observations and this served not only as a template for further exploration, but also clarified his thoughts about the phenomenon he was interested in. Writing this way serves two purposes: one is to record the observation at the moment of exploration and other is to seed new thoughts and questions that can be derived out of these recordings. You can surely get a lot out of this approach – give it a try.

Coming back to the sky – what is the exact origin of its colour? It took almost 400 years after Leonardo’s observations for someone to come up with an ‘accurate’ answer. And that person was Lord Rayleigh (actual name: John William Strutt). In his remarkable research paper published in 1899, Rayleigh explained the blue of the sky as due to molecular scattering. The opening paragraph of this paper is historic and reproduced below-

“This subject has been treated in papers published many years ago. I resume it in order to examine more closely than hitherto the attenuation undergone by the primary light on its passage through a medium containing small particles, as dependent upon the number and size of the particles. Closely connected with this is the interesting question whether the light from the sky can be explained by diffraction from the molecules of air themselves, or whether it is necessary to appeal to suspended particles composed of foreign matter, solid or liquid. It will appear, I think, that even in the absence of foreign particles we should still have a blue sky.”

The final statement is significant as it recognizes that light scattering can occur purely due to molecules in air, even after discounting the contribution of suspended particles. Rayleigh gave his famous formula in 1871, which drew an inverse relationship between the intensity of scattered light from a very small particle (compared to wavelength of light) and the fourth power of the wavelength of light. In other words, smaller the wavelength of light (violet-blue in case of visible light), more will it be scattered from the molecules in the sky, and hence the blue colour.

One may wonder why not a violet coloured sky. After all, if the inverse relationship between scattering intensity and wavelength holds good, then according to visible colour distribution (VIBGYOR), violet should be the dominant colour of the sky. The answer to this puzzle is a complex one. Mainly because what we perceive as blue is due to a combination of at least three concomitant effects: Rayleigh scattering, human perception and the background in which the scattered light from molecules are observed. Although the colour of the sky is beautiful to perceive from earth, the intricate understanding of the optical processes in atmosphere of planets, including earth, is still a work in progress.

The foundations laid by Leonardo opened a new line of thought, and Rayleigh put forth an important explanation that forms the basis for a majority of studies on light scattering since 1900s. We will revisit Rayleigh and his work many times in this blog; meanwhile enjoy watching the sky with your scientific eye – after all sky is no limit for science!

Beginning of scattering

Why a new blog, now ?

Well, I have been pondering for sometime to write about my research on light scattering. One of the main motivations of why I became a student of science was to understand scattering of light. Be it the blue of the sky, the flying comets in space or a glowing molecule in a biological cell, light scattering has something to say on everything in this universe. This ubiquity of a physical process is worth exploring, and has lead me to take this intellectual journey. In an essence, I was, and continue to be fascinated by the beautiful concept of how light scatters off matter of various scales, be it the size of galaxy or be it a tiny little atom. Over the past 15 years or so, light scattering has been integral to everything I have been working on, and I think it is high time that I share the beauty of this subject through the posts in this blog. I intend to do this by exploring various aspects of light scattering – from fundamental theory to mind boggling applications.

History has a role – The field of light scattering itself has a very rich history, dating back to observations of Leonardo Da Vinci, spectacular opti’k’s by Newton, marvelous explanations by Rayleigh, creative experiments by Raman, connections by Tyndall, conditions by Kerker and so on….in fact the list is endless, and the story is compelling, which has to be said. As we take this journey, we will visit the masters, pick their brains, ask questions, and pester them for answers. This process, I promise, will be rewarding, and will hopefully keep you interested…

Lab stories – Another reason for starting this blog is to emphasize the experimental techniques in area of light-scattering research. Most of the times, the hard-fought battles of experimental laboratory researchers in unveiling the truth and beauties of nature goes unrecognized and under-represented, especially among general audience of science. Although, Einsteins and Maxwells of the world, deservingly get a lot of attention and applause for their theories, people like Bloembergen and Askins don’t get their share for the spectacular experiments they have performed. This blog, in way, is to compensate for such discrepancies.

Unity in diversity – An ulterior motive behind this venture is also to explore new aspects of light scattering in physics, which is evolving and taking new shape as I write. In this discussion about new physical concepts of light and matter, I wish to highlight its relevance and connection to various branches of science and technology. We will evidence how sub-branches of sciences progressed due to ideas and applications from light scattering. In this context, let me give you two examples: soft-matter physics and radiative transfer in astrophysics. A tremendous amount of information about nature of soft-matter and interstellar matter has been derived from light-scattering theories and experiments. As you may notice, the scales of these problems are tremendously different, but the underlying physics is essentially the same. In this blog I intend to showcase this unity of concepts in a diversity of problems in science and technology, how light scattering takes a center stage in this play.

Ultimately, what excites me to share this story is that I get to be a student all over again. This is a happy place to be for a professor: it keeps you grounded and engaged. After all, in the process of every learning and expression their is a concomitant enlightenment and scattering of thought. This is my intellectual kick to scatter forward…..