Peacocks Don’t Fade: The Science of Structural Colours

Explore how Aksharas boundary-defying physics project led to the inclusion of a new lab component in a biology course.

Feat pic

Richard Fernandes was not remotely puzzled by the sight of peacock feathers in his office when he walked in one morning. What else could it be but a gesture of gratitude from his student Akshara Yagnik who had successfully completed her research project titled Structural Colouration in Peacock Feathers”? 

For Akshara, this was the culmination of a year-and-a-half of scientific deliberations and experimentation— a journey whose seeds may have actually been sown much earlier. As a kid, even before I really knew what it meant, I wanted to be an astrophysicist,” she said.

It wasn’t just the allure of black holes and planets, Akshara was also intrigued by the colours of these celestial objects. Later, in tenth grade, she was introduced to the concept of pigments. I learnt that this pigment in leaves called chlorophyll absorbs all the colours except for green, and that’s why it releases green light.” 

This piece of information stayed with Akshara and added a new layer to the way she viewed the world. She gazed at the wooden laminated walls of her classroom wondering if there was a similar selective absorption of light causing it to appear brown. Whether it was art class or a visit to a plastic factory, she found herself constantly amazed at how we were able to manufacture so many specific colours. She wondered how these pigments know which colours to absorb and which to reflect.

Naturally, when the time came to think of potential honours project topics for her BSc in Physics at Azim Premji University, Akshara thought of delving into the physics of colour. Tipped off by her mentor Proteep Mallik, she signed up for a webinar by Nipam Patel, an Indian-origin biologist at the Marine Biological Laboratory at the University of Chicago. This was when Akshara came to know about a special type of colouration found in nature. 

Most of the reds, yellows, blacks and browns found in the wings of butterflies are due to pigments, explained Nipam. These work just like the chlorophyll molecules in a leaf, selectively absorbing light to make leaves appear green. 

On the other hand, the greens and blues in many birds, animals and insects occur through a different mechanism that doesn’t usually involve pigments. Nanostructures present in these organisms cause light waves to interact in ways that manifest as brilliant colours that seem to blend and transform depending on the angle of viewing. These kinds of colour, produced not by pigments but by tricks of light, are called structural colours.

Structures in the Junonia species of butterflies give it its brilliant colours. 

Image Credit: Rachel Thayer

Akshara was smitten by this idea of structural colouration and subsequently became convinced that this was a research topic that would sustain her interest over the next couple of years. 

Along with Proteep, Richard Fernandes (a physics faculty member who is no longer at the University) and Aahana Ganguly, they began devising a setup to measure colour. They tinkered with various mechanical pieces to build something that could hold samples and allow them to measure angles and detect light. They received a shot in the arm thanks to the biology group that happened to procure a mini spectrophotometer, a device that made it convenient for Akshara to measure how much of a particular wavelength of light a sample was reflecting at each point of its surface.

Conversations with Divya, a biology faculty member at the University, reaffirmed with Akshara that what she wanted to do was study structural colour in nature but through the lens of physics rather than biology.

The mini spectrophotometer wasn’t the only way the biology group contributed to Akshara’s project. Right at the early stages of the project’s conception, her mentor Proteep had encouraged her to get in touch with Divya Uma, a biology faculty member at the University who he knew was interested in colours in nature. Behavioural ecologists like Divya are well aware that for biological entities, being colourful often comes at a cost. 

To produce a pigment, certain biochemical pathways may need to take place, or the pigment may be secondary metabolites of certain reactions. Whereas structural colours are just byproducts of the structure, it seems zero-cost in terms of energetics…” she pointed out. 

Conversations with Divya reaffirmed with Akshara that what she wanted to do was study structural colour in nature — like Nipam Patel, but through the lens of physics rather than biology.

Some examples of structural colours in nature include (clockwise from left) bee-eaters, jewel beetles, marble berries and golden mole. 

Image Credit: Wikimedia Commons

With Divya’s help, Akshara got to know which samples would be easily available for them. They tried the metallic blue-green or purple wings of a carpenter bee, and the exoskeletons of the jewel beetle that shimmer green, purple and sometimes orange. These were both enigmatic examples of structural colouration in nature, however, they proved tricky to manœuvre and measure efficiently. 

It was only a matter of time before Akshara realised that the ideal sample was right in front of her. My house in Gandhinagar, Gujarat, has peacocks just roaming around. Peacock feathers were abundant!”

Fortunately, peacock feathers proved much more convenient to work with. Light would fall on the feather at a constant angle while I moved around the mini spectrophotometer. We could observe iridescence very clearly,” she said.

With Divya and Proteep’s help, Akshara formulated her research question. She summed it up: A peacock feather has a white central stem which we call the axis. If you twist it along this axis, you can clearly see that the central part of the feather changes between green and blue. As you go further outside it, the colour changes between green and yellow, and then between green and pink. We wanted to have a more quantitative understanding of this colour change.”

To make the measurements that would enable her to answer this question, Akshara relied on a custom-built variable angle holder that Richard and Proteep helped her set up. They focused on the most central part which shimmers between green and blue as the viewing angle (or reflection angle) increases. 

They measured the wavelengths of the light being reflected and found that the peak reflectance at 20 degrees was obtained at around 500 nanometres (green), and this peak shifts to 450 nanometres (blue) as the viewing angle increases to 70 degrees. [Note: This iridescence was quantified at a constant incidence angle of 45 degrees.]

These results made sense, and Akshara was able to prove that the optical mechanism behind colouration in peacock feathers was not diffraction, but interference.

A variable angle holder screwed together using laser-cut pieces from acrylic sheets in the physics lab. 

Image Credit: Akshara Yagnik

In this manner, Akshara was able to experimentally demonstrate something that Robert Hooke had noted in his book Micrographia (1665) and Isaac Newton in his book Opticks (1705). It was a whole century later that Thomas Young declared that wave interference was the principle behind this phenomenon.

Though it has been over three centuries since structural colours were first documented, there are still many questions to be answered. Most of what we know so far emerged from experiments that used expensive equipment such as scanning electron microscopes and more sophisticated spectrophotometers. What we are doing,” emphasised Proteep, is at orders of magnitude less cost, but obtaining results with similar fidelity.” 

These days, there is an increasing interest in structural colours because they present an alternative to chemical-based colours that dominate our manufacturing today. If you expose pigments to too much light, then they will just essentially bleach and become white,” pointed out Proteep. 

This does not happen in the case of structural colours. Divya added that structural colouration is also being seen as a possible way out of our over-reliance on dyes, which cause significant chemical pollution. 

Akshara, Divya and Proteep pose with peacock feathers in the lab. 

Image Credit: Akshara Yagnik

With the rigid divides between physics, biology and chemistry in conventional science education in India, such projects are rare. They open up the possibility of creating new and exciting labs.”

Her project might have concluded but the imprint it made on Akshara and on the University’s science programme may be everlasting. It piqued her mentors’ interest in structural colour, and it has spurred on other spin-off multidisciplinary projects — for example, another one of Proteep’s students Pranav is growing photonic crystals with specific lattice structures. 

Divya is especially heartened by the boundary-defying potential of such topics. With the rigid divides between physics, biology and chemistry in conventional science education in India, such projects are rare, she pointed out. They open up the possibility of creating new and exciting labs.” 

And it’s already happening! Akshara’s project led to the design of a new lab component in the introductory biology course where students learn how to quantify colour. Meanwhile, she has begun her further studies in the area of photonics, which deals with the physics of light waves. I am now doing a Masters in Photonics at Abbé School Of Photonics in Germany,” she updated us happily over an email.

About the Team

Akshara Yagnik is a BSc Physics (2020−2023) graduate from Azim Premji University.

Divya Uma, Proteep Mallik and Aahana Ganguly are faculty members at Azim Premji University. Richard Fernandes is a physics faculty member who is no longer at
the University.

Nandita Jayaraj, the author of this article, is a Science writer and Communications Consultant at Azim Premji University.

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