The Incredible Science Of Color & Light: Structural Coloration


Color is by the common man term for the reflection of certain wavelengths of light bouncing from an object that then strikes the retinas in your eyes to which your brain then proceeds to process as certain colors depending on the signals you receive. Though many have this basic grasp of the concept, the resulting colors after light hits certain things is not quite as clear as you may think for there are many other mysterious functions at work, here we will cover the phenomena of structural coloration.


Structural coloration in the most broad and superficial idea is the

production of colored by using microscopic formations small enough to filter and interfere with the separate wavelengths of light that produce the different ranges of visible light. In the electromagnetic spectrum there is a small portion devoted to the range of colors with blue as the shortest (400nm) and red as the longest (700nm) wavelengths of light. These structures are fine enough to allow certain wavelengths through while reflecting the other back and therefore producing the smaller wavelengths of colors. This is important to discuss because it is vital to many of the colors produced within the animal kingdom for many animals – if not all – can only produce a limited range of pigments to create a small range of colors. The 1665 documentation “Micrographia” by Robert Hooke, a natural philosopher in London England, describes the finer structures on the feathers of a colorful peacock reflect different colors under observation. A peacock’s feathers contain only brown pigments but due to its microscopic structural configuration, they appear in many cool colors and even appear to show signs of iridescence.


A strong concept to emphasize – that will likely be explained many times – is that the effect/trickery of the light stops working once you reach smaller and smaller scales. For example, the wings of a morpho butterfly appear blue but once you zoom in enough, the surface of the wings begins to loose saturation and their brown pigmentation is thus revealed.
Structural coloration is not the end of the line in the explanation of the awesome colors created by nature, instead the rabbit hole continues as reality grows yet more mysterious as we dwell in the many mechanisms and methods used to refract and reflect light.


Diffraction Grating

Isaac Newton and Hooke both studied the peacock’s feathers using light microscopes and inferred that the light was generated by ways of interference; however, their light microscopes were simply not powerful enough to dive deep enough to solve the mystery. The polymer C8H12O5N or chitin is a primary building ingredient in crustaceans, insects and other arthropods with exoskeletons. Some of those creatures can use diffraction grating to split and diffract (this is when a wave is split upon contact with an obstacle) light into several beams traveling in many directions. This is why the wings of flies and flying insects do not have a single color but rather a mix of colors. One thing that I will point out many times is that most colorful animals are unable to produce colors in the cool range, meaning the blue, green and purple end of the visible light spectrum. Instead these animals must manipulate light by ways of diffraction grating, in the case of a Morpho Butterfly, which is held as an animal with one of the highest strengths of blue hues. When a highly magnified look is sent onto the wings, it will be revealed that on the microscopic surface there are many scales like structures that diffract and interfere with the incoming light to produce the recognizable deep blue wings. It does this by allowing certain wavelengths of light to be absorbed while reflecting the ones it wishes to appear certain ways. There is a certain range of effectiveness that once exceeded; the trick of the light no longer works, though this range requires a high powered microscope.

 

Iridescence
Compact Discs are not made from the same material but have tiny grooves which act as diffraction grating and are involved in the very colorful spectrums of colors that you see on the bottom of CD’s when you tilt and move them around in the light. Spiderwebs also show diffraction at work along with iridescence – which will be covered soon enough – to lace its web with rainbows. Iridescence is the physical property that causes surfaces to appear differently depending on the angle of which you view it. There are many other examples such as the necks of pigeons, or the surfaces of bubbles. The variating rainbow of colors is caused when some of the incoming light reflects off a certain surface and some of it passes through. If the light that passes through reflects again, it comes back at a different angle. Depending on whether or not it comes back in or out of phase – which is when the waves align, like when how in choruses the basses all sing together to create a stronger sound wave and is thus the result of constructive interference and are in phase. When the light waves reflect back out of phase, then different strengths of reflected light bounce back creating an uneven and changing – depending on the angle of view and shifts in the surface of the bubble or whatever is giving the iridescent property – will be visible. This property is also the reason why sometimes after rainy days, some of the puddles on roads appear colorful with lots of shifting colors. The water is mixing with the oils of passing cars which create separate layers of reflective surfaces that the light bounces off at different angles and thus creating in and out of phase light waves just as the above description shows. The reason why it is not a single bold color is


that the oil concentration and levels are not uniform and thus create various colors of the rainbow. This effect also affects city pigeons that have the colorful necks – which never grew on me, I’ve also thought they were unappealing – that appear differently depending on the angle of view. Constructive interference is also why they sometimes appear very bright when they move around and about.

Selective Mirrors
Another effect due to the hands of chitin; selective mirrors are microscopically sized bowls that reflect 2 or more different wavelengths of light that then combine to form color mixtures. For this example we will borrow another glorious butterfly the Emerald Swallowtail butterfly otherwise known as the emerald peacock. This is a black butterfly with a green V-cut into its wings. The green color is produced by mixing blue and yellow light that is then perceived by the human eye as green. If you’d look at the visible light spectrum, you’d see that green is located in between just like in a color wheel you’d learn in elementary school. You may take note, that this method also does not directly produce green light, but instead accomplishes this by illusion and tricking the brain in thinking that it is seeing green light. This form of color mixing – without actually overlapping the colors to produce green – is also taken advantage of in most computer monitors and TV screens. For example, when you see a yellow light on your screen, it is not really yellow. Instead it is simply the act of the pixels on your screen placing red and green bars very closely packed together. A magnification of a yellow screen would reveal thousands of green and red lines paired next to one another. In fact, you may begin to see this effect if you look at your monitor closely enough. This phenomenon is denoted as RGB, with red green and blue. These are the colors needed to create all of the other colors and this combination is used in all screens and monitors.

Adaptive Structural Coloration
Those are many of the static structures used in nature and in engineering, but some of Mother Nature’s life being is capable of taking it a step further and controlling it to their will. The Hawaiian bobtail squid uses reflectin proteins in the cells in its skin to manipulate the incidental light (incoming light) to camouflage itself from danger. This recently classified protein group of reflectins can be activated with electrical signals that can rearrange them to create different colors both statically and adaptively to hide itself. Some cephalopods also show signs of iridescence in their skin and close ups show the famous rainbow of colors that you see on CDs.


This covers the basics of structural coloration, but like all of science where there is discovery comes more questions both fundamental and complementary to the discovery. Along with finding more examples and how common this method is used in the animal kingdom and why it exists and how it even came into existence. There is much more to be covered – the certain animals who take advantage of structural color – but that will be saved for another time.
 

Sources Cited
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Article Written By GDop26

RIT student

Last updated on 28-07-2016 352 0

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