Category: optics

Good books : Bohren & Huffman

Cover of ‘Absorption and Scattering of Light by Small Particles’ by Craig F. Bohren and Donald R. Huffman.

It is important to read good books. Astrophysics, quantum mechanics, and gravity (including attempts to combine them with quantum mechanics) have been at the forefront in terms of popular physics imagination. These are wonderful subtopics of physics, but there are a few others that need equal emphasis. So, here is my attempt to fill this gap with some book recommendations.

The first one in the optics community is just called ‘Bohren and Huffman’ and is one of the best technical books I have read and continue to read. It is humorous and filled with wonderful insights that still engage researchers and students alike.

Craig Bohren, a theoretical physicist, is a wonderful writer, and you will see more of his books discussed here.

The book introduces the scattering matrix from a ‘light scattering’ viewpoint, and has a direct connection to laboratory measurements.

Humour is one of the key aspects of this book (as with others from Bohren), and the title of chapter 8 gives a nice glimpse:
“A Potpourri of Particles”

There is a famous section in Chapter 11 with the heading – “Extinction = Absorption + Scattering” that wonderfully explains the physics behind it.

Overall, an outstanding book for understanding optics from an electromagnetics viewpoint and also to learn how electromagnetism is harnessed to understand interactions at the classical spatio-temporal scales.

Read this if you are interested in physics…It is a delight!

In audio-visual form:

Where Ideas Merge: A Visit to the Institute of Science Tokyo

With Prof. Daiki Nishiguchi

New ideas are often created by the merging of two old ideas. How often is this true, and how often do we tend to forget this?

Today I visited the Institute of Science Tokyo, formerly known as Tokyo Tech. This is a new avatar of a very interesting institution funded by the government of Japan. By merging the Tokyo Institute of Technology with the Tokyo Medical and Dental University, a very interesting concept has emerged: the Institute of Science Tokyo. These two institutions have been important pillars of the research and educational landscape of Tokyo, and I had the privilege of visiting this new place, which is a result of a new merger.

Thanks to the invitation and fantastic hospitality of Prof. Daiki Nishiguchi, a faculty member in the Physics Department of the Institute of Science Tokyo, I had a memorable experience. I met Daiki a couple of years ago at the University of Tokyo, where he previously held a faculty position. Recently, he has moved to the Institute of Science Tokyo to establish his independent research group as an Associate Professor.

Daiki has done amazing work on topological soft matter, and his recent results include remarkable observations related to turbulence and vorticity in suspensions of bacteria under spatial confinement. He has also been setting up interesting experiments involving Janus particles, and I got a nice overview of his work. Thanks to him and his research group, I got a flavor of the research being carried out in their lab, and I was also treated to a wonderful lunch by Daiki.

I gave a physics seminar on some of our work on structured light and confinement of soft matter, especially thermally active colloidal matter in optothermal potentials. Since Daiki and his group (see image below) have expertise in topological soft matter, my seminar emphasized structured topological beams, including ring optical beams and optical vortices. I gave an overview of our experimental results and highlighted the prospect of utilizing the topology of light to interact with topological soft matter.

There is much to explore at this interface, and again, it brings me back to the point that new ideas often emerge from the merging of evolving old ideas, such as topological light and topological soft matter.

This is my third visit to Japan, and I always find their calm, focused, and deeply committed research environment inspiring. There is much to learn from their approach to science and technology, and my visit to the Institute of Science Tokyo reinforced this thought.

I thank Daiki and his research group for the wonderful time I had at their laboratory and offer my best wishes to him in his new explorations.

Floral colours, CV Raman and illustrations

In the 1960s, C. V. Raman wrote a series of papers on floral colors and the physiology of vision. In there, he was very interested in the origin of colors from various different flowers. This was also motivated by his fascination with optics and natural colors in vegetation. Specifically, during that era, he had a large garden at his institution and he was deeply immersed in understanding the origin of the colors from these wonderful living creatures. 

By using his knowledge of spectroscopy and the chemistry of pigments, he was able to explore some of the spectral features of the floral colors. The diagrams that you are seeing are illustrations from his paper published in 1963.

As you can observe, these illustrations are beautifully created. I don’t know whether Raman himself drew these pictures, but one should really appreciate the artist who has created them.

In a broader sense, it also indicates two important aspects. The first is that Raman was deeply motivated by natural phenomena. His intuition of optics helped him to understand the origins of a variety of natural optical processes. Spectroscopy was a crucial element in all the things that he did. The second aspect is that, in a deeper sense, aesthetics is interwoven with the pursuit of science and Raman’s work, especially towards the later part of his life, showcased it. 

There is a fascinating video conversation with Richard Feynman where he describes the appreciation of the beauty of flowers by a scientist. Raman’s appreciation of beauty is close to what Feynman is describing in the video.

C. V. Raman was a curious person. He had a deep inclination to explore natural phenomena, using the knowledge and tools he had accumulated over several decades. In that sense, he was a scientist driven by curiosity before and after his Nobel prize.

Next time when you see a flower, remember that it is a creature of beauty and science merged together.

ps: blogpost in audio-visual format

Optical computing – review link and a few thoughts..

“How might optical computers beat electronic computers? …….. There are three main metrics of computing performance for which we might aim to achieve an advantage: latency, throughput and energy efficiency…”

A very readable review by Peter MacMahon of Cornell.

In the immediate future, designing energy-efficient computational platforms will be a necessity. Electronic transport is noisy and dissipative. Optical alternatives can be important, but challenges remain…

Given that the speed of light is the upper limit of information transport and processing, optics will be a vital ingredient in computation. In hindsight, it has already been. But there is more to it than just the speed, as the review article explains elaborately..

Interesting times ahead…

Herschel, IR radiation and the 3 steps of science

I came across a nice article on the discovery of Infra-red (IR) radiation by William Herschel. It has some information on how he detected the IR part of the electromagnetic spectrum.

It was interesting to note that Herschel’s viewpoint after writing a few papers on this topic was to conclude with uncertainty on the interpretation of his own result.

One may argue that he was not confident in his work, but to truly discover something new, one will have to be at the boundary of the unknown. At such an inflection point, uncertainty and self-criticism are so important, and I don’t blame Herschel for being circumspect.

Nevertheless, his work received attention in spite of his doubts and motivated other researchers, such as Ritter and Röntgen, to explore the extended spectrum of electromagnetic radiation.

Scientific progress is two steps forward and one step backward. The backward step (as in questioning new findings, verification, clarification and debate) is one of the hallmarks of scientific thinking, without which we may take many forward steps, albeit in a totally wrong direction.

Hints Before a Discovery: The Case of Neutrons

In the late 1920s, the quantum theory of matter was still under construction. Questions such as what are the constituents of a nucleus of an atom were pertinent. The then understanding was that a nucleus was made of protons and electrons. Yes, you read it right. People thought that electrons were part of an atomic nucleus. But those were the times when quantum theory was evolving, and many ‘uncertainties’ persisted. For example, if one considered a nitrogen nuclei, it was postulated that it had 14 protons and 7 electrons.

The other important aspect during that time was the question of spin statistics, including that of nuclei, which was under exploration. The classification of nuclear spin in terms of Fermi-Dirac statistics or Bose-Einstein statistics was under study, and researchers were trying to sort it from theoretical and experimental viewpoints. Going back to the example of nitrogen nuclei, it was categorized to obey Fermi-Dirac statistics.

Enter Rasetti and his Raman Spectra

With this backdrop, let me introduce you to Franco Rasetti. Franco was an Italian physicist who was visiting Caltech in the US on a research fellowship. This was 1929, and many exciting quantum thoughts were in the air. C. V. Raman had just discovered an inelastic scattering process in the visible frequency spectra of molecules, and there was interest in understanding the quantum nature of the interaction between light and matter. Motivated by this, Rasetti set up an experiment at Caltech to probe Raman spectral features of molecular gases. Rasetti was an elegant experimentalist and, later, went on to become a close associate of Enrico Fermi and played a crucial role in the nuclear fission experiment[I].

Coming back to the work of Rasetti’s experiment, he took up the problem of understanding the Raman effect in diatomic gases (nitrogen and oxygen) and wrote a series of papers[ii]. Among them, he published his observations on rotational Raman spectra of diatomic gases (nitrogen and oxygen). Specifically, he performed a series of experiments and recorded beautiful spectral features of rotational lines of diatomic nitrogen. In that work, Rasetti discussed the specific selection rules from an experimentalist viewpoint and identified that the even lines in the spectral features were much more intense compared to the odd lines.

Guys from Gottingen

During the same year, working in Göttingen as postdocs of Max Born were Walter Heitler and Gerhard Herzberg. These two gentlemen studied the paper of Rasetti with great interest and went on to write an interesting paper in Naturwissenschaften (in German)[iii]. The translated title of the paper was “Do Nitrogen Nuclei Obey Bose Statistics”.

In that paper, Heitler and Herzberg studied rotational Raman features of nitrogen molecules and compared them to hydrogen molecules. They considered arguments based on the symmetry of the eigenfunctions and associated them with statistics (Fermi or Bose). Hydrogen nuclei obeyed Fermi-Dirac statistics, whereas Nitrogen counterpart did not. From the analysis of symmetry, they found that Rasetti’s observation contradicted the convention that Nitrogen nuclei obey Fermi-Dirac statistics. So, with this contrast, they clearly indicated that the nuclei of nitrogen should obey Bose-Einstein statistics, provided Rasetti’s experiments were correct.

How was the discrepancy resolved?

In 1932, James Chadwick[iv] went on to discover the neutron, and this discovery laid the foundation for understanding the constituent of a nucleus. With the new observation, one had to account for the presence of neutrons inside the nucleus. In the context of Nitrogen nuclei, it was found that it contained 7 protons and 7 neutrons, and the total spin was 1 (in contrast to spin ½ of hydrogen). This ascertained that Nitrogen nuclei obeyed Bose-Einstein statistics and removed the discrepancy from the observed rotational Raman spectra of Rasetti.

3 takeaways

What can we learn from this story? The first aspect is that experiments and theory go hand in hand in physics. They positively add value to each other and connect the real to the abstract thought. Second, it emphasizes the importance of careful observations and their interpretation. The third lesson is that Rasetti, Heitler and Herzberg were all young people and learned from each other’s work. They were essentially post-docs when they did this work, and we are still discussing it today.

Sometimes, good work has a long life.

[i] https://en.wikipedia.org/wiki/Franco_Rasetti

[ii] https://www.nature.com/articles/123205a0

https://www.pnas.org/doi/epdf/10.1073/pnas.15.3.234

https://www.pnas.org/doi/epdf/10.1073/pnas.15.6.515

[iii] https://link.springer.com/article/10.1007/BF01506505

[iv] https://en.wikipedia.org/wiki/James_Chadwick

Gold nanoparticles in sync – preprint

We have a new preprint: https://arxiv.org/abs/2411.15512

The central circle indicates anchored gold nanoparticles stuck to the glass, and the two moving circles are gold colloids that are trapped synchronously due to the optothermal potential.

Gerhard Herzberg – scientific life


References:

Pavan Kumar, G. V. “Gerhard Herzberg (1904–1999): A Pioneer in Molecular Spectroscopy.” Resonance 29 (2024): 1339. https://www.ias.ac.in/describe/article/reso/029/10/1339-1345.

Stoicheff, Boris. Gerhard Herzberg: An Illustrious Life in Science. Ottawa : Montréal ; Ithaca N.Y.: Canadian Forest Service,Canada, 2002.

Stoicheff, Boris P. “Gerhard Herzberg PC CC. 25 December 1904 – 3 March 1999.” Biographical Memoirs of Fellows of the Royal Society 49 (December 2003): 179–95. https://doi.org/10.1098/rsbm.2003.0011.