by Pavan Kaushik – It was a cold, misty morning and the sun was just about to peek out of the spiky horizon. I had gone on a birdwalk in a tiny hilly town called Rakkar, 20kms of Dharamshala. The mist that settled on the previous night was demistified by the warmth of the sun. The rising fun finally gleamed off the snow clad Dhauladhar mountain range to give it a golden aura. My literary skills are miniscule to luridly elucidate the phenomenon of how the sun in high latitudinal regions during the winter rises like a gentle giant, slowly but steadily, benevolently giving its ways of warmth, to which the ice at the peaks gleams with a bittersweet squeak and cries away as it flows down on a long journey of being a life giver, reservoir of waste, toxins and pathogens, be damned at a dam ever so often and hopefully reaches its mother before the gentle giant picks it back up and resets the cycle to start over again.
I was lucky to see the journey of water at its start. A tiny stream perennially flowing, surging and receding with the rhythmic dance of the sun and clouds albeit with a delay (the snow mountains are much further than you think it is). As the stream streamed its music to the air, it also sent rippling waves to mark the start of a performance given by the orchestra of birds around. Bulbuls, prinias, shrikes filled in their voices. Vultures soared in the rising thermals. Hoverflies hovered in their coveted sun slivers. Lizards warmed themselves on the dark rocks.
As I walked along the stream, away from my usual city life, stuck in a dark room, probing the brains of tiny flies on how they decide what to do in life, there was a ray (of hope?). Beacons of light gleamed at me. There was an interplay of light and the water surface. The gentle giant had finally exposed his grand scenery. While I stared at the glistening stream, I saw spots on the stream bed. A leaf was touching the surface of water and due to surface tension, there was a depression in the water surface. The parallel rays of the sun broke through the water surface and landed on the stream bed. But those that hit the depressed region bent away and landed slightly off target. The curvature caused the water to act like a lens and it created spots on the stream bed. One can see chromatic aberration (suggesting it to be a not a great lens) albeit, it is still a lens. If you look at the curved surface on the water, there is the magnified image of the stream bed. One can also not unsee the conservation of energy in action. As the light gets focussed into a tiny region, the other regions get darker than usual and if you compare the average intensity at the flat region against the curved region, the bright and dark cancel out to be equal to the gray.
I walked further down the moving spots. I looked around for leaves, tiny particles and saw nothing. I realized then that, what I see is not the leaf but the depression of water surface. Not only surface tension, but also a vortex can cause a depression. I looked for tiny tornadoes but I hit a rock wall. Then it hit me, when you hit a rock wall, you have eddies. The calm, steady stream had hit a rock and in its wake, created a von karman vortex street. The slow-mo video shows the elaborate process of vortex generation, propagation and destruction. The idea of using parallel rays passing through different refractive index is the starting premise of Schlieren imaging. The difference could be due to the difference in temperature/pressure/density and makes the invisible turbulence around us visible.
We ended up using the same technique in the Shoot for Science workshop which I was fortunate to be a part of. Our group had to visualize the research done by Kruthika on the ecology of insect gut microbes. And we were stuck in a challenging problem of visualizing microbes on a leaf surface. The shadow lighting technique not only visualized the microbe colonies but also made it a pretty appealing cover image. Who knew that shadows, dark themselves could be bright enough to illuminate us from the abyss of ignorance. What bright shadow did you see today?