G V Pavan Kumar

Everything..

This is a reverse-chronological list of all the blogs posted on this site.

Tiny value of the radiation pressure of sunlight on Earth

If radiation pressure is indeed a genuine electromagnetic phenomenon, then why don’t we observe it in our everyday lives?
The reason is that the magnitude of the radiation pressure from the natural light source on Earth (the Sun) is feeble.
from electromagnetic theory, this tiny amount of pressure can be calculated by the formula \(\frac{E}{c}\), where ‘E’ is the energy of sunlight on earth and ‘c’ is the speed of light in vaccum (which is \(3 \times 10^8 \text{ ms}^{-1}\) [\(9.83 \times 10^8 \text{ ft s}^{-1}\)]).
Maxwell himself recognized the low value of this energy, which he assumed to be \(83.4 \frac{\text{ft} \cdot \text{pound}}{\text{sec} \cdot (\text{ft})^2}\)
Taking this value and dividing it by ‘c’ gives us a radiation pressure of \(10^{-7} \frac{\text{pound}}{(\text{ft})^2}\).
Poynting, who extensively worked on radiation pressure from an electromagnetic theory viewpoint, compared this tiny pressure to the size of a grain in an area of \(200,0000 \text{ (ft)}^2\)!
This highlights why radiation pressure is hard to measure experimentally, and it took some trial and error to ascertain the value and the method. More on this later…

Reference :

Loudon, R., and C. Baxter. ‘Contributions of John Henry Poynting to the Understanding of Radiation Pressure’. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 468, no. 2143 (2012): 1825–38. https://doi.org/10.1098/rspa.2011.0573.

Posters & Thinking…

Conversation with Prasad Subramanian

Prasad Subramanian is a Professor at the Indian Institute of Science Education and Research Pune whose research spans plasma astrophysics, solar physics, and space weather. His work focuses on solar coronal mass ejections, black hole accretion, and astrophysical jets, combining theoretical approaches with observational data. He has contributed significantly to understanding turbulence, magnetic fields, and energetic processes in cosmic plasmas. He is also actively involved in science communication and interdisciplinary academic initiatives.

In this conversation, we discuss general aspects of astrophysics and the research topics he is interested in.

References with links:

‘Prasad Subramanian’. Accessed 21 May 2026. https://sites.iiserpune.ac.in/~p.subramanian/.

‘Prasad Subramanian – IISER Pune’. Accessed 21 May 2026. https://www.iiserpune.ac.in/research/department/physics/people/faculty/regular-faculty/prasad-subramanian/343.

‘‪Prasad Subramanian‬ – ‪Google Scholar‬’. Accessed 21 May 2026. https://scholar.google.co.in/citations?user=EHu_h-kAAAAJ&hl=en.

‘Nptel.Ac.in/Courses/115106124’. Accessed 21 May 2026. https://nptel.ac.in/courses/115106124.

When Chandra’s paper got rejected

Sometimes, referee reports can be frustrating, especially if your paper gets rejected and criticized without justification. This is not a new thing in scientific discourse, and even accomplished researchers like S. Chandrasekhar had to face such rejections. As Chandra notes in the winter of 1956:

“The frustration of these months was due also to the fact that the Royal Society rejected my second paper on turbulence with a most discourteous referee’s report. I withdrew the paper, but continued the correspondence with the referee. The referee withdrew some of his more blatant remarks; but the whole incident was an unhappy interlude. I went specially to Washington to talk to von Neumann; and corresponded also with Heisenberg.” (Chandrasekhar, 2010, p. 38)

When a paper gets rejected, what is important is to seek feedback from people who are knowledgeable and courteous. Chandra had friends such as von Neumann and Heisenberg to seek input. One cannot get better than this.

Source: Chandrasekhar, S. 2010. A Scientific Autobiography: S. Chandrasekhar: With Selected Correspondence. (posthumously published)

Optical Momentum – Lectures

My latest research grant from ANRF is on “Opto-Thermal Binding of Plasmonic Matter”. This is a topic that is at the interface of optical momentum, thermodynamics, statistical physics, and advanced optical microscopy in the real and momentum spaces.

Optical momentum and its measurement have a rich history in understanding electromagnetic waves and their interaction with matter. Over the past century, multiple applications have emerged that harness the transfer of momentum from light to matter. Interestingly, the light that is scattered off this interaction also carries relevant information not only about the interaction but also about certain parameters of light and the participating matter.

These lectures are my attempt to give an overview of the field. My main target audience is my PhD group members and senior undergraduates who are working with me. But these lectures can be followed by anyone who is seriously interested in physics. The discussion involves theoretical optical physics (including elements of statistical and quantum optics), experimental techniques (including advanced microscopy methods) and a few computational techniques connected to the interaction. The goal of the lectures is to reveal the interesting questions in research papers, review articles, monographs and conference papers related to the field and their possible application in industries, including biophotonics and astro and space-photonics. From time to time, I will also discuss our research results from the project.

C1, C2 and life…

Dear Young People,

Below is a brief lesson that I share with those who ask me for career advice. I hope it will be useful to you.

Category 1 (C1): Honesty, humility, discipline, effort and compassion

is as important as

Category 2 (C2): Irreverence, creativity, drive, communication and risk-taking.

In this era, C1 may look boring and sound cynical, but C1 can give you as much of a competitive advantage as C2, especially in the long run.

Importantly, C1 will build your character. C1 is probably the best way to build self-confidence, and hence self-respect.

This is not to underplay C2, which is equally important. Just that C1 and C2 are not zero-sums.

C1 and C2 are incomplete lists, and you can always expand them and keep referring to them for directions in life.

Best Wishes,
from a fellow student

Feynman’s birthday with a Raman diagram

Chandra quotes Virginia Woolf

The well-known astrophysicist, S. Chandrasekhar, liked the writings of Virgina Woolf. In her words, he found a unique channel to philosophize his own work, as he did in 1957:

‘By accident, I found the following quotation from Virginia Wolff (Woolf) which expressed very accurately my attitude to my work of the past years. This quotation ends my Rumford Lecture.

“There is a square. There is an oblong. The players take the square and place it upon the oblong. They place it very accurately. They make a perfect dwelling place. The structure is now visible. What was inchoate is here stated. We are not so various or so mean. We have made oblongs and stood them upon squares. This is our triumph. This is our consolation.”’ (Chandrasekhar, 2010, p. 41)