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 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).”
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.
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.
“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 !
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.
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!
Delhi airport : A peek from the widow of the flight at around 9.30 in the morning
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: PM10, PM2.5, NO2, SO2, CO, O3, NH3, 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”.
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.
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!
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…..
VISMAYA - History & Philosophy of Science 