Nature

Polarised light in the animal kingdom

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A property of light that is invisible to humans is routine to help animals get about. Birds are known for remarkable quality of vision. They have larger eyes, as a proportion to body size, than other animals, and the eyes are adapted, for a wide field of view range, for grazers, or sharp focus, for birds of prey. The colour-sensitive nerve endings too, in the eyes of birds, fish and reptiles, are specialized and of four kinds, against single kind that we have, and this helps some of them see in the ultra-violet too. Insects have compound eyes, which help them cover a very wide angle of view by each eye and detect very fast movement. And then, some snakes have ‘pits’, which are like eyes and can detect the warmth of prey in pitch darkness.

But a most remarkable feature of birds and insects is that they can sense a property of light called polarization, which helps them deduce the position of the sun, even if it has just set or is covered by clouds. In fact, some birds can even tell the position of the moon, when it is hidden by clouds, with the help of this property of light, in moonlight! But the best that human eyes can do is to tell whether the light is bright or dim, and the only use we make of the property of polarization of light is in special sun-glasses, to cut glare due to reflected light.

The journal Nature Communications carried a report by Stefan Greif, Ivailo Borissov, Yossi Yovel and Richard A. Holland, scientists at Belfast, Seewiesen,  Germany and Tel  Aviv, that they have found that bats too, can make out the polarization of light, and this helps the calibrate their magnetic compass, so that they can navigate at night (1). Bats use sound waves to detect and pinpoint prey, but they need other means to know their way about and get back to their roosting place after a night out in search of food. That bats make use of the polarization of light is a new discovery, as the ability has not been noted in mammals so far.

The greater mouse-eared bat. Credit: Wikimedia Commons.

The greater mouse-eared bat. Credit: Wikimedia Commons.

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The luminous life of plants

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“There are lots of dimensions of light that we cannot see with our eyes,” NASA Earth scientist Joshua Fisher explained, gesturing towards a couple of olive trees, “That’s the interesting science NASA does.” It was the third day of spring and we were sitting at a picnic table in the shade at NASA’s Jet Propulsion Laboratory in Pasadena, California, yakking away about fluorescent light, which plants emit during photosynthesis.

Light travels in waves. The human eye is adapted to see a small range of those waves in the visible part of the electromagnetic spectrum. A few NASA instruments, such as MODIS on the Aqua and Terra satellites, the Landsat suite of satellites, the Mars Reconnaissance Orbiter and Cassini, just to name a few, make observations in the visible part of the spectrum. But NASA has also created sensors specifically designed to pick up light waves outside of the visible spectrum. These instruments can observe additional electromagnetic wave energies, from the Cosmic Microwave Background radiation left over from the Big Bang to high-energy gamma rays, and everything in between, and help us understand more about Earth and the universe.

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Perceiving Light – the trickiest biological application on Earth

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One of the most important abilities developed by living organisms on Earth is adaptation to the light that comes from the nearest star in our galaxy – the Sun. This ability is called light perception. Interestingly, it defines not only the perception of a source of light, but the perception of all surrounding reality! It determines colors, shapes, orientation in space and in time.

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Memoir of Light

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Next time you look at something, find the source of the light that you’re seeing. Most probably it’s not coming from the object you’re looking at. Visualize the two stories associated in everything that we see, the story of the object, and the story of the light that reaches our eye. The odds are that you never paid attention that there is a hidden memoir attached to every light we see, a history that can be much more than whatever object it represents to us.

Think of the light from the moment when it has been created. Look at a candle light, it shines through the room and when its light hits an object, let’s say a book, we see the book, because it is that light beam’s last stop before reaching our eyes, but most likely not the only stop. We “see” the book; we see, where it is and how it looks like, but what about the story behind the light itself? Where it has been made? How far and how many reflections it has traveled through before reaching our eyes? In most cases, the light we see from our surrounding is a reflection of the light that was generated somewhere else, from a light source, perhaps a candle, flashlight, lamp, or the Sun.

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Rainbows, when the light becomes poetry

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First and Second order Circular Rainbow in Nyalam, Tibet 2014. Credits: Antigone Marino

This picture was taken in Nyalam, Tibet. It shows what it seems to be two “circular rainbows”, is that even possible? Keep reading. Credits: Antigone Marino

This summer I was traveling in Tibet. It was a hot day, and we were moving from the Tibetan Plateau down to the Kathmandu Valley in Nepal, jumping fast from 5,000m to less than 1,500m altitude. We had a stop in the Nyalam, a small Tibetan town of the Shigatse Prefecture near the border, situated at 3,750 meters above sea level. When my sister, with an astonished voice asked me “What’s that up in the sky?”

I had not time to answer as I was already pointing up with my camera to catch the first and only circular rainbow I have ever seen. A few seconds after, with my face still hidden from the camera, I was trying to build up a possible, realistic and credible physical explanation of that phenomenon, not to lose the credit of “all you can answer” optician in front of my sister.

That’s something you have to face every day when you are a scientist: people can ask you whatever they want, whenever they want.

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