Science Of Butterfly Colours

As winter leaves, spring arrives with blossoming flowers and plenty of butterflies dancing on them. Seeing their colourful wings we wonder how these colours were created by nature.

We all know about the presence of scales on the wings of a butterfly. These scales arranged in an intricate pattern give rise to bright colours by a combination of various phenomenon of light (absorption, reflection, interference, diffraction and refraction). Scales on butterfly wings are made of chitin, a semitransparent polymer of glucose derivative, assembled on a pigment layer (eg. melanin) which gives the wing its base colour.

Scales of Polyommatus daphnis (Lycaenidae)

reference: Z. Vértesy et. al. 2004

Absorption and Reflection: Pigments of various colours are present below the chitin layer. These pigments absorb some colours from incident light and reflect remaining colours. A combination of reflected colours forms the base colour of butterfly wing.

Interference and Diffraction: Scales on butterfly wing, act like a grating, cause dispersion of light (A grating is a surface with many parallel grooves that splits light into its constituent colours). The constituent colours of the incident light then undergo interference and diffraction. Interference is the superposition of two or more waves that results in a new wave pattern. Interference of colours can be constructive or destructive in nature. Constructive interference raises the intensity of colour where as destructive interference lowers the intensity of colour. Diffraction is the bending of waves around small obstacles and the spreading of the waves past small openings.

Refraction: As mentioned above the chitin forms a semitransparent grating (also referred as photonic crystals) which is also capable of causing total internal reflection. Certain constituent colours of white light undergo total internal reflection in these photonic crystals. The remaining constituent colours that do not undergo total internal reflection gives rise to brightest colours of butterfly as they emerge from the wing. Same phenomenon is also responsible for brilliance of a diamond.

All the colours of butterfly wings are not only due to the above-mentioned phenomenon but also due their combined effect. Colours emerging due to refraction may further undergo interference and get brighter. Some butterflies have two layers of chitin, which may further give rise to intricate combination of colours. Next time when we see a butterfly, just ponder how amazing is nature. The beauty of butterfly is because of a chitin layer that has no prominent colour of its own but gives rise to so many colours due to its translucent nature.

Nuclear Physics - 1

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1. Great Discovery, Humble Beginning…

The 19th and the 20th centuries were the time for the most breath-taking discoveries and inventions of modern science. What was once considered fiction and everything that was ever dreamt of - flying machines for carrying people non-stop from continent to continent, submarines which could travel under water from Pole to Pole even under ice, rockets to carry us to the other worlds in the universe, apparatus to make it possible to converse over long distances without wires, and what not.

The development of science and technology outran the fantasies of the writers and the dreams of the scientists. One of the miracles of the era was the discovery of a mysterious chemical, a matchbox full of which could produce enough energy to propel a large ship for several years! The secret to its vast energy lies deep inside the matter that surrounds us.

At the turn of the 20th century, little was known about the structure of matter. Not all elements had been discovered, however it had been established that all matter was made of atoms. Atoms were believed to the smallest, and hence indivisible, particles of matter. J J Thomson then discovered the electron, the smallest particle of negative charge and soon Robert Millikan determined the mass of an electron to be 1836 times lighter than an atom of hydrogen, the lightest of all elements. In 1898, Thomson proposed that the indivisible atom was a uniformly distributed positively charged sphere, in which electrons were embedded. This proposal couldn't answer several of the questions raised about the plausibility of positively charged particles, stability of the atom and so on.

Becquerel's Mistake

The phenomenon of the luminescence of certain substances when exposed to sunlight is called fluorescence. The French scientist Henri Becquerel spent many years studying this phenomenon. Once he had observed a photographic film wrapped in a black paper and kept in a drawer was exposed. There was no way this could have happened because the substance (sulphate salt of potassium and uranium) he used could have fluoresced in the darkness of the drawer. When he studied more carefully the reasons for the same, he could establish that the binary salt of uranium and potassium emitted invisible rays that could expose the photographic film even in darkness. Thus, 26 February 1896, marked the discovery of a new physical phenomenon which became the starting point of the whole of new physics of the 20th century. It is interesting to note that all of the physics that followed started from this accidental observation. More to come in the articles to follow…

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Nuclear Physics: An Overview

0. Nuclear Physics – a fascinating subject

E=mc² ... In class 9, I was taught this simple equation. This is one of the most popular equations of science and it was given by none other than Albert Einstein. What does it mean? It mathematically tells us that mass (which you can see or feel) can be converted to energy (which cannot be seen or felt) and vice-versa. This was given as a part of Einstein's Special Theory of Relativity which talks about things becoming longer, clocks slowing down and many more counter-intuitive and very interesting ideas.

Everyone might have heard about the nucleus. The nucleus has protons (positively charged) and neutrons, but how are they held together? Protons should be repelled by each other, isn't it? They are held together by an enormous force called “Nuclear Force”, which is several times stronger than the Coulomb force of repulsion between the protons. Existence of such a force also implies the presence of an enormous amount of energy. Where does this energy come from? Some of the mass of the nucleus is converted to provide this energy. Nature is clever! The mass lost in the conversion is termed as mass-defect and the energy produced is called binding-energy. More the binding-energy per nucleon more stable the nucleus is (find out the most stable nucleus). It is this binding energy which is of interest for us.

Humans are trying to tap this vast energy using two types of nuclear reactions called nuclear fission and nuclear fusion. Nuclear fission reaction is used in modern day nuclear reactors and the fuel used is uranium, plutonium or thorium. The atomic bombs dropped on Hiroshima and Nagasaki was based on the same principle. Nuclear fusion reactions have so far been out of human reach. Though we have made the H-Bomb or the Super Bomb which works on this, we are unable to control it and put it to better use. The sun gains all its energy from this reaction. Scientists have been trying to replicate that reaction in a controlled manner. India is a member of the seven member group working on a project called ITER (International Thermonuclear Experiment Reactor) working on these lines. The ITER program is anticipated to last almost 30 years and cost over $13billion, which makes it one of the most expensive modern techno scientific mega projects.

There are also other areas of modern day research going on; these include particle physics, nuclear structure, hadron physics, neutrino physics, nuclear astrophysics and so on. All these began as a part of nuclear physics but today they are vast subjects. They are many research facilities working round the clock in these areas. To name a few CERN, KEK, FERMILAB, Argonne, GSI, SLAC, DESY, Brookhaven, Budker INP, JINR and CEA. Every year billions of dollars are being spent on research in these fields. Can't we put the money to better use is a question always raised. Everything we discover has its pros and cons; it is for us to make a judicious use of the available technology.

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Live Colours

LIVE COLOURS

Looking around me, I find thousands of colours in nature. Various shades of green leaves, yellow flowers, multi-coloured butterflies, fishes etc! None of the shades in my colouring-set can give the exact hue of the flowers in the garden. So how does nature bring about such a variety (hundreds of shades in red, blue, green etc) of never fading, live colours??

The answer lies in the tiny cells (typical animal cell: 10µm to 30µm, plant cell: 10µm to 100µm) which make up all living organisms. Body of every living organism has its share of colour or pigment producing cells called ‘chromatophores’. So first of all what is a ‘colour’, it is nothing but light of a particular wavelength, which falls within the visible spectrum of sunlight (390nm to 780nm). Pigments are compounds which selectively absorb certain wavelengths of light and reflect the rest, which our eyes can perceive as specific colour of the object.

The most common plant pigment is ‘chlorophyll’, which is synthesised within special organelles inside a plant cell called ‘chloroplast’. It is chlorophyll which impart green colour (by strong absorption of red and violet colours of incident sunlight) to the leaves and are tools for trapping solar energy during photosynthesis. Other common plant pigments are carotenoids which give yellow or orange colour (eg: xanthophyll, they act as antioxidants and also assist in photosynthesis by absorption of blue light not easily absorbed by chlorophyll). Anthocyanin gives purple blue colour, seen in black grapes, and has nutritional benefits. Among animal cells, melanin is the pigment responsible for colours of skin (among human races), hair, iris of the eye, feathers of birds etc. Melanin is dark brown or black in colour, but depending on its chemical state can impart other colours like red (pheomelanin). Melanin producing cells are called ‘melanocytes’. It absorbs ultra violet (UV) rays (280nm to 400nm) and protects the cell from UV induced damage. Greying of hair, albinisms etc are result of reduction in melanin synthesis (find out more about albinism!). Thus colours not just make our world look alive, but are essential for the living beings to carry out various biological processes.

Coming to occurance of numerous shades, it happens due to different combinations in which pigments are produced. Further differences in concentration leads to absorption of different regions of visible spectra and thus reflection of rest, giving rise to very subtle differences in colours perceived by our eyes!

Sneha