Spectral Colours, Pigments, Tyndall Effect & Interference. This is part II of my ongoing series about colour. Read also part I, Colours I : The Nature of Colour 1.
All Visible Colours are not Pure Spectral Colours
It is wrong to believe that all chromatic colours we can perceive are pure spectral colours; there are also mixed spectral colours. That include colours not appearing on the colour wheel, such as brown and various brownish colours, which can only be obtained by mixing spectral colours. It also includes colours that - in a perspective of a colour wheel – are found between violet and red, while optically they do not exist. This is best described by quoting my explanation of the difference between violet and scientific purple in “Purple Light? It Doesn't Exist! And what is Visual Purple?”
“Scientifically there is a difference between violet and purple, and the order from red to blue is red - purple - violet – blue. Violet is colder than purple, and if the intensity of light increases, violet becomes more and more blue, which is not the case with purple. This definition does not always coincide with cultural definitions.
When we talk about light, there is no “in between red and blue”. Red and blue are the opposite sides of the spectrum. The pure spectral colours are red, orange, yellow, green, blue, and violet. If we continue outside the visible spectrum, we get infra-red on the red side, and ultraviolet on the violet side. It's not as if red and blue would meet there somewhere, as they would in a colour wheel.
We can mix red light and blue light and get purple, but that's a mixed colour, not a spectral colour. Optically it would remain mixed red light and blue light, not purple light. Physically, or optically, there is no such thing as purple light. It does not exist.
In the visible spectrum, there is a (pure spectral) colour with shorter waves than blue, but before we reach the end of visibility. When the wavelength is one step shorter than the visible spectrum, we have ultraviolet. The colour in the spectral interval between blue and ultraviolet is called “violet”. This is in accordance with common practice. Violet light is an optical reality, it has its own wavelength."
Pigments
Most colours are a result of pigments. A pigment is a chemical compound which selectively absorbs and reflects certain parts of the spectrum.
A red pigment, for instance, reflects red light and absorbs all the others. So what we see is red.
Pigments and other colouring matters can be mixed, with well-known results, like blue+yellow=green, red+yellow=orange, etc.
Although many pigments are now made synthetically, nature is full of them, some with very important biochemical functions for plants and animals.
Structure
Colours can sometimes be a result of structure, like the Tyndall effect, and interference.
Structural (non-metallic) colours are caused by reflected light scattered by small particles. If the particles are very small, they scatter more of the short waves of light (blue, violet) than by the long waves (red). The result is a reflection of blue light.
When the short waves of sunlight are reflected by the earth, and then scattered by molecules of oxygen and nitrogen in the atmosphere, you get a phenomenon you can see every day (unless you have very bad weather): a blue sky!
Blue eyes of humans and other animals are based on the same principle. Very small proteins in the iris scatter the short waves of light and the reflected blue light is seen against a black layer of melanin further back in the iris.
Albinos get reddish eyes because they are lacking the layer of melanin, so the blue colour stays concealed by the blood of the capillaries of the iris.
In brown eyes, the blue colour is concealed by occurrence of melanin among the scattering particles.
This structural blue is called Tyndall blue, and the phenomenon causing it is called the Tyndall effect. (Physicists might call it Rayleigh scattering.)
Structural colours with a metallic shimmer are mostly a result of interference. This is best understood if you watch a thin layer of oil on a water surface. The light is reflected by the surfaces of the oil and the water, and the waves of these are not in phase. The result can be that the waves either enforce one another, or reduce, or even neutralise one another, so that only the opposite wavelengths remain - and they are seen as a colour. In fact you get a rainbow-like effect, since the distance from the surface of the water and through the oil differs, depending on the angle from which you are observing it.
To be continued...
(This article in an excerpt from the introduction of my forthcoming e-book, "Chromaticon", a book exploring colour in culture and nature. Parts of it are based on material previously published in Meriondho Leo.)
Copyright © 2010, 2021 Meleonymica/Mictorrani. All Rights Reserved.
(The lead image shows colours caused by interference. Photo by mumi chen/Pixabay, CC0/Public Domain. The image has been cropped.)
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Interesting. I didn't know much about it. Then I would like you to talk about those who have different colored eyes, and their physiological reasons why it happens.