{"id":39825,"date":"2018-10-17T19:58:42","date_gmt":"2018-10-17T19:58:42","guid":{"rendered":"http:\/\/www.labri.fr\/perso\/barla\/blog\/?p=39825"},"modified":"2020-04-19T20:04:23","modified_gmt":"2020-04-19T20:04:23","slug":"the-physics-and-chemistry-of-color","status":"publish","type":"post","link":"https:\/\/www.labri.fr\/perso\/barla\/blog\/?p=39825","title":{"rendered":"The Physics and Chemistry of Color"},"content":{"rendered":"<p id=\"top\" \/>\n<p><em>by Kurt Nassau<\/em><\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Some fundamentals: color, light and interactions<\/h3>\n\n\n\n<p>The three subtractive primary colors &#8211; magenta, yellow, and cyan &#8211; are complementary to the three additive primary colors. If we remove the red-orange region from the full spectrum, we obtain cyan. Removing the violet-blue region gives yellow, and removing green gives magenta. To predict subtractive color mixing, it is necessary to use the individual absorption spectra. &#8211; p.17<\/p>\n\n\n\n<p>It is usually assumed that an object appears colored because part of the incident light is<strong> absorbed during the process of reflection at the surface<\/strong>. This actually does occur in an indirect way in metals,&nbsp; and in some very deeply colored opaque substances, such as graphite. <strong>Substances of this type maintain their color when in powdered form.<\/strong> More usually, the light penetrates somewhat into the object and is<strong> absorbed within the object before being reflected; powdering will result in the loss of color with this type of material.<\/strong> &#8211; p.28&nbsp;&nbsp;<em>The top layer of a material might be porous, foamy, or powdery; it will then most of the time be color-less for the reasons above.<\/em><\/p>\n\n\n\n<p>Most substances that we meet in everyday life are not single crystals. Nevertheless, <strong>most solid substances, including metals, ceramics, rocks, and so on, are polycrystalline<\/strong>, consisting of small particles, each of which is a single crystal. &#8211; p.29<\/p>\n\n\n\n<p>It might seem remarkable that so many distinct causes of color should apply to that small band of electromagnetic radiation to which the eye is sensitive, a band less than one &#8220;octave&#8221; wide in a spectrum of more than 80 octaves. So much happens in this narrow band because this is the region where the interaction of radiation with electrons first becomes important. <strong>Radiation at lower energies induces relatively small motions of atoms and molecules, which we sense as heat, if at all. Radiation at higher energies has a destructive effect<\/strong> since it can ionize atoms, that is, completely remove one or more electrons, and can damage molecules permanently. Only in the narrow optical region to which the human eye is sensitive is the energy of light well attuned to the electronic structure of matter, with its wide diversity of colorful interactions. &#8211; p.31<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Color produced by incandescence<\/h3>\n\n\n\n<p><strong>Incandescence<\/strong> is produced by any material solely because it is at a high temperature, so that the <strong>atoms and molecules emit part of their energy of vibration as photons<\/strong>. With increasing temperature, <strong>the color sequence black, red, orange, yellow, white, and blue-white is produced<\/strong>. Although actual incandescent objects can deviate from the ideal black-body incandescence curves, it is always possible to assign a color temperature, this being the temperature of a black-body which to the eye most closely matches the color. &#8211; p.47<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Color produced by gaz excitations<\/h3>\n\n\n\n<p>An atom in the gas or vapor phase can have its electrons excited into higher-energy orbitals and can then re-emit part of this energy as visible light. The electronic configuration of the atom leads to <strong>the possible energy levels, and the selection rules<\/strong> establish which transitions are allowed; this combination <strong>determines the wavelengths<\/strong> of the photons emitted. <strong>Flame colors, neon tubes, fluorescent lamps, arcs, lightning, coronas, auroras, and gas lasers all involve gas excitations.<\/strong> &#8211; p.67<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Color produced by vibrations and rotations<\/h3>\n\n\n\n<p><strong>In both liquid water and solid ice<\/strong> there is a strengthening of the bonding over that in an isolated H2O molecule by the formation of <strong>hydrogen bonds<\/strong>. This is a relatively rare type of bonding that here produces cross linking between two water molecules. The resulting absorption spectrum of water is quite complex, consisting of a series of strong narrow bands in the infrared. These bands rapidly become weaker in approaching the visible, as is usually the case with high overtones and combinations. There is, however, <strong>a significant amount of absorption remaining at the red end of the visible region<\/strong>, and the complement of this red absorption is a blue color. &#8211; p.76<\/p>\n\n\n\n<p>The hydrogen bonding in ice is similar to that in water, and the color of pure bulk ice is also a pale blue. This color is seen particularly well in the solid ice exposed in ice caves in glaciers and in icebergs. Here again, <strong>light absorption is derived from bond vibrations. A green color in ice usually indicates the presence of algae as in water.<\/strong> &#8211; p.77<\/p>\n\n\n\n<p><strong>Vibrations and rotations provide additional energy levels<\/strong> to each electronic energy level, which modify the absorption and emission spectra. Examples include the violet color of iodine vapor, the green of chlorine gas, and the <strong>blue emission of some flames.<\/strong> &#8211; p.78<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Color produced by transition metals in a ligand field<\/h3>\n\n\n\n<p>The best-known cause of color is that derived from <strong>transition metal compounds or impurities<\/strong>. This provides the color of many of our <strong>minerals, gems, ores, paints, and pigments<\/strong>. It involves <strong>inorganic compounds containing metal ions<\/strong> <strong>with unpaired electrons in d or f orbitals.<\/strong> &#8211; p.83<\/p>\n\n\n\n<p>In the formation of sodium chloride, both ions have only completely filled or completely empty shells, and all electrons are paired off. Such paired electrons require a very high energy to become excited; so large, in fact, that <strong>only radiation well out in the ultraviolet is energetic enough to be able to unpair and then excite such an electron. Accordingly, pure table salt cannot selectively absorb visible light and is, therefore, colorless.<\/strong> Similarly, all electrons are paired off in a <strong>pure diamond crystal<\/strong>, which is colorless. &#8211; p.84<\/p>\n\n\n\n<p>If we view this arrangement from an ionic point of view, the aluminum ion is surrounded by six negatively charged oxygen ions, O2-. These six charged neighbors produce an electrostatic field at the aluminum ion, called the crystal field, the nature of which can be specified by describing the symmetry arrangement, distorted octahedral in this case, and the overall strength of the electric field. From a more general point of view, recognizing that bonding is never purely ionic, it is possible to treat this same situation as a ligand field case. Here <strong>not only the electric charges but also the effects of the specific bonding characteristics of the ligands, the ions or molecules surrounding the central atom or ion, are included<\/strong>. The symmetry and the strength of the ligand field are now the controlling factors, quite analogous to the crystal field picture. &#8211; p.86&nbsp;<em>I don&#8217;t clearly understand these two alternative explanations.<\/em><\/p>\n\n\n\n<p>A corundum crystal composed of <strong>pure aluminum<\/strong> oxide is a rather uninteresting-appearing material also known as white or colorless sapphire. When it contains 1 % or a little more of chromium sesquioxide (Cr203), so that about <strong>1 of every 100 aluminums is replaced by a chromium<\/strong>, the material acquires a <strong>beautiful luminous red color and is known as ruby<\/strong>, a gemstone material rivaling diamond in status. &#8211; p.86<\/p>\n\n\n\n<p><strong>Two absorption mechanisms occur when white light passes through a piece of ruby<\/strong>: 2.2-eV light can be absorbed to take the chromium from the 4A2 ground level to the 4T2 excited level, and 3.0-eV light takes it to the 4T1 level. The first corresponds to absorption in the yellow-green part of the spectrum, and the second to a violet absorption. <strong>Because of vibrational interactions these are broad absorption bands rather than sharp lines<\/strong>; they overlap somewhat, so that the <strong>transmission in the blue is rather small<\/strong>. In the red region below 2 eV the absorption falls completely to zero, thus giving to ruby its red color with a slight purple overtone. &#8211; p.89<\/p>\n\n\n\n<p>In moving from ruby, the epitome of all things red, to <strong>emerald<\/strong>, having a hue so distinctive that only &#8220;emerald green&#8221; will describe it, we are discussing a color caused by the same impurity, chromium. Not only that, but the chromium in emerald is in the same valence state, Cr3+, as in ruby and even in the same type of environment, again replacing Al3+ and again being at the same distance from six aluminums in an octahedral arrangement distorted in a manner very similar to that in ruby. Where, then, is the difference? Because <strong>other oxides are present in the structure besides aluminum oxide, namely beryllium oxide and silicon oxide, the overall bonding is a little weaker, that is, the ligand field is less strong.<\/strong> &#8211; p.92<\/p>\n\n\n\n<p>With such a drastic change in color in going from ruby to emerald resulting from a relatively small change in the ligand field, one might wonder what would be the result of a ligand field intermediate between that of emerald and that of ruby. Nature has provided for us an answer to this question in the form of the extremely rare and precious <strong>gemstone alexandrite<\/strong>, an answer that demonstrates how she can confound our expectations and yet turn out to be perfectly reasonable in retrospect.<strong> In blue-rich daylight<\/strong> or the similar-quality light from a fluorescent lamp we see an <strong>intense blue-green color<\/strong>, somewhat resembling emerald, while <strong>in red-rich candle light <\/strong>or the light from an incandescent lamp we perceive a <strong>deep red color<\/strong>, somewhat resembling a ruby. <strong>Nature has found a way of avoiding the almost impossible task of providing a color truly intermediate between the green of emerald and the red of ruby!<\/strong> &#8211; p.93<\/p>\n\n\n\n<p>When pieces of tourmaline are placed, anyone of which was green by transmitted light, on top of each other, the transmitted light color becomes first yellow, then orange, and finally, red as the thickness is increased. This phenomenon (a color change with thickness) is closely related to both the alexandrite effect (a color change with light source) and the dye dichroism (a color change with concentration) and occurs also in plastics. &#8211; p.94&nbsp;<em>It is called the Usambara effect.<\/em><\/p>\n\n\n\n<p>A distinction is made between those compounds in which the transition element is a major and essential ingredient, also called idiochromatic (&#8220;selfcolored&#8221;) in mineralogy, and substances colored by small amounts of transition elements present as impurities (sometimes called dopants, activators, or chromophores); these latter can be also designated allochromatic (&#8220;other colored&#8221;) compounds. A rapid and fairly reliable way to distinguish these two groups is to examine the color of the powder or perform<strong> the mineralogist&#8217;s streak test by rubbing the sample across a porous ceramic plate<\/strong>. Most <strong>colors caused by impurities are much less saturated<\/strong> than might be judged from their appearance, and the color will no longer be seen in the powder or streak, whereas most idiochromatic substances will maintain their color even when finely divided. &#8211; p.102<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Color in organic molecules<\/h3>\n\n\n\n<p>Chromophores include carbon-carbon double bonds, particularly in conjugated systems containing alternating single and double bonds as in the carbon chain structure. A <strong>light absorption near the ultraviolet edge of the visible spectrum is shifted by auxochromes<\/strong>, which may be bathochromic or red shifting (shifting to a longer wavelength), hypsochromic or blue-shifting (shorter wavelength), hyperchromic (increasing the intensity of the color), or hypochromic (decreasing the intensity). Fortunately, <strong>auxochromes, which are bathochromic, also tend to be hyperchromic, thus providing both intensification and the desired shift to longer wavelength (lower energy)<\/strong>. &#8211; p.114<\/p>\n\n\n\n<p>In transition metals, the 4f transition elements have narrow absorption and emission lines rather than bands because the 4f orbitals are shielded from the ligand field by 5s2 and 5p6 orbitals. In the 3d transition elements the 3d orbitals are less well shielded by the 4s and 4p orbitals, resulting in medium-width absorption bands. In<strong> organic compounds<\/strong> the bonding leading to color involves mostly p orbitals, which are <strong>unshielded from electrostatic interactions<\/strong> and accordingly frequently produce <strong>very broad absorption and fluorescence bands<\/strong>. &#8211; p.120<\/p>\n\n\n\n<p>Polyenes involving cyclic but nonbenzenoid conjugated systems include the porphyrins, the most important of which are chlorophyll and heme (or hemin or haemin). These two molecules, so alike in many respects, supply, respectively, the green pigment that converts the light absorbed by plant leaves into chemical energy by photosynthesis and the red pigment in mammalian blood that transports oxygen. The action of evolution can clearly be seen at work here. &#8211; p.123<\/p>\n\n\n\n<p>To produce yellow, orange, and red colorants, a single absorption band in the blue-violet-green region suffices. Similarly, a single absorption band in the red-orange-yellow region will produce violet and blue. A central blue-green-yellow absorption will produce purple. <strong>Only for the greens are two absorption bands required<\/strong>, producing difficult problems for the designers of organic colorants. &#8211; p.127<\/p>\n\n\n\n<p>The vast majority of organic dyes can be viewed as containing an extended conjugated chromophore system to which are attached electron donor and electron acceptor groups. In discussing the absorption of light by vibrating atoms, it was necessary to look for lighter atoms and shorter, stronger bonding to increase the frequency of the vibrations to move the absorptions from the infrared into the visible. Here, however, the opposite applies, since we must move the absorptions from the ultraviolet into the visible by lowering the frequency. This can be done by increasing the size of the conjugated system, which can be achieved directly by extending the size of the chromophore or by adding donors or acceptors that further extend it either by direct conjugation or by involving non-bonding p orbitals so oriented that they can interact strongly with the pi system. Additional electrons can also be pumped into the conjugated system from donors. &#8211; p.129<\/p>\n\n\n\n<p>Most fascinating is the combination of the &#8220;blue&#8221; and&#8221; green&#8221; plastics. At 1 mm each, the blue and green combine to give a cyan, a bluish green, as might normally have been expected. With 2 mm each of &#8220;blue&#8221;and &#8220;green&#8221; plastic, the result is a deep red similar to that of 4 mm of the &#8220;green&#8221; by itself. &#8211; p.136&nbsp;<em>This is the Usambara effect once again!<\/em><\/p>\n\n\n\n<p>The <strong>larger and more complicated organic molecules can absorb light into energy levels involving molecular orbitals<\/strong>. These are generally based on pi bonding in a conjugated system of alternating single and double bonds, which may have attached to it electron donor and acceptor groups. This covers most biological colors, including <strong>animal and vegetable colorations and natural as well as synthetic dyes<\/strong>. &#8211; p.141<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Charge transfer color<\/h3>\n\n\n\n<p>A crystal of corundum containing a few hundredths of 1 % of <strong>titanium <\/strong>is colorless. If, instead, it contains a similar amount of <strong>iron<\/strong>, a very pale yellow color may be seen. If <strong>both impurities<\/strong> are present together, however, the result is a magnificent deep blue color, that of <strong>blue sapphire<\/strong>. The process at work is intervalence charge transfer, the <strong>motion of an electron from one transition metal ion to another<\/strong> produced by the absorption of light energy; this results in a temporary change in the valence state of both ions. &#8211; p.143<\/p>\n\n\n\n<p>Charge transfer produces exceptionally intense absorptions because the transitions are fully allowed by the selection rules.<strong> Colors are usually brown, dark blue, or black<\/strong> for intervalence charge transfer, involving the transfer of one electron between two variable-valence transition metal ions, either of one type on two different sites or of two types on the same site. <strong>Blue sapphire and many of our pigments, including Prussian blue, are examples of intervalence charge transfer, as are most of the yellow, brown, and black colors of iron-containing rocks and minerals.<\/strong> Other types of charge transfer involving metal-ligand, anion-anion, and donor-acceptor interactions lead to the color of chromates. permanganates, lapis lazuli, and graphite. &#8211; p.152<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Color in metals and semi-conductors<\/h3>\n\n\n\n<p>The high electrical and thermal conductivity as well as the high reflectivity of metals is derived from the occurrence of empty electron states in the energy band at essentially any energy above the Fermi level. A high reflectivity leading to a metallic appearance is also seen in other strongly absorbing surface color materials, including those with delocalized electrons such as &#8220;fool&#8217;s gold&#8221; (pyrite) and graphite. &#8211; p.184&nbsp;<em>Probably the most difficult color source to understand&#8230;<\/em><\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Color centers<\/h3>\n\n\n\n<p>An electron color center occurs when an electron is present in a location where an electron is not normally found. Contrasted to an electron color center is a hole color center, where an electron is missing from its normal position to produce a light-absorbing center. &#8211; p.192<\/p>\n\n\n\n<p><strong>Color centers involve the displacement of an electron by irradiation or other techniques with the formation of a hole center; the displaced electron becomes trapped to form an electron center<\/strong>. Either or both of these centers can be the color center that absorbs light. Unstable color centers are bleached by light or heat, stable ones only by heat. Some color centers act in a reverse manner, being formed in the dark and being bleached by light. Similar-appearing color changes can occur in materials involving transition metal ligand field color without, however, the formation of hole and\/or electron centers. &#8211; p.203&nbsp;<em>Thus<strong> color centers and ligand field colors are the only sources of time-varying colors<\/strong>, unless one considers different layers of materials where one color is revealed when the top layer vanishes somehow.<\/em><\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Dispersive refraction and polarization<\/h3>\n\n\n\n<p>In the region of absorption, the natural resonating frequency of the light absorbers interacts with the vibration of the light in a complex manner involving the phase velocity and the phase angle to produce a speeding up of the light, thus giving a lower n value on one side of the absorption and a slowing down and a higher n value on the other side. In the region of the absorption, the refractive index increases with the wavelength, instead of decreasing; this may be difficult to observe since it occurs just where the light is most strongly absorbed. For a detailed treatment of the dispersion it is necessary to employ the complex refractive index. If either the refractive index variation or the coefficient of absorption variation is known for all wavelengths, the other one can be calculated by using the Kramers-Kronig dispersion relationships. <strong>We usually tend to think of the absorption as the &#8220;cause&#8221; and the dispersion as the &#8220;effect,&#8221; but the two are inextricably connected, and one cannot exist without the other.<\/strong> &#8211; p.214<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Scattering and non-linear effects<\/h3>\n\n\n\n<p>In a gas, liquid, or a glass the atoms and molecules are evenly distributed on a macroscopic scale, yet at the atomic level there is considerable non-randomness. As one example, individual molecules, as well as small clusters of a few molecules in a gas or a liquid coming together in collision for a brief instant before dispersing again, will act as light-scattering particles much as do particles of dust. In a glass there will be similar density and refractive index variations, both from the imperfect mixing of the various ingredients of the glass as well as from the frozen-in liquid fluctuations. Even in what might be thought of as a perfectly ordered single crystal, there usually will be <strong>a variety of point defects (impurity atoms, vacancies, and clusters of these) and line and plane defects (dislocations, low-angle grain boundaries, and the like) as well as density fluctuations from the thermal vibrations of the atoms or molecules, all of which scatter light. <\/strong>&#8211; p.233<\/p>\n\n\n\n<p>Scattering becomes more intense and much more prominent in the forward direction; it is also much less strongly dependent on the wavelength but in a more complex manner. The scattering is no longer predominantly blue but sometimes shows different colors, mostly red and green bands, only at certain angles derived from the position of lobes, called polychroism. This effect can be seen only if all the particles are exactly the same size, since otherwise overlap results in an overall whitish scattering. <strong>With larger particles only white scattering is observed<\/strong>; it is this white color that we see when we look at the water droplets that constitute<strong> fog, mist, and low clouds.<\/strong> &#8211; p.234<\/p>\n\n\n\n<p><strong>The red color is intensified by fine dust particles low in the atmosphere<\/strong>; this contribution has <strong>more effect at sunset than at sunrise<\/strong>, since more dust is produced during the day than at night, both from human activities and from the drying out of moist earth and wind. <strong>Volcanic eruptions <\/strong>which inject large quantities of fine particles into the upper atmosphere can produce<strong> exceptional deep red sunsets.<\/strong> &#8211; p.235<\/p>\n\n\n\n<p>The clear sky is not the only place to see blue scattered light. Air pollution yields the same phenomenon, whether produced by man or by jungles and forests. The latter generate large quantities of &#8220;<strong>summer heat haze<\/strong>&#8221; or &#8220;natural smog.&#8221; This is emitted by all vegetation and is composed of aromatic organic volatile substances of the terpene family, which are then oxidized in the air to form <strong>tiny droplets of tars and resins<\/strong>. Names such as the Blue Ridge Mountains are derived from this effect, which is absent only in large deserts or over the oceans. We use this phenomenon automatically to estimate the distance of faraway mountains and are misled when <strong>clean air, occurring after a rain has washed away the pollutants, makes the mountains appear much closer<\/strong>. &#8211; p.235<\/p>\n\n\n\n<p>Among mammals there are a number of Tyndall blue occurrences, particularly in skin and eyes. <strong>Many monkeys have brilliant blue and purple areas in their faces, buttocks, and genital areas; the blue is derived from melanin-backed skin scattering<\/strong>, while the <strong>purple involves scattering combined with red reflections from hemoglobin<\/strong> in blood vessels close to the surface. Then there are the variously colored nevi or birthmarks, including the purple &#8220;portwine&#8221; blemishes and the &#8220;Mongolian spot&#8221; purple sacral patch. In all such instances the purple color turns blue after death in the absence of hemoglobin as the blood is withdrawn from the surface circulation. &#8211; p.238<\/p>\n\n\n\n<p>Surprisingly, there is a significant blue-scattering component to the skin of pale-colored Caucasians; a freshly shaved person with a pale skin and heavy dark facial hair will show a bluish sheen from scattering in the surface layer of the skin backed by the dark hair just below the surface. Similarly, veins show as blue because they provide the dark backing to surface scattering. Pale Caucasians turn blue when cold because the surface capillary blood circulation which provides the red component of the pink color is shut down to conserve heat. The same happens after death; if the skin is then water soaked, the scattering structure is destroyed and the color becomes white. &#8211; p.238<\/p>\n\n\n\n<p>The color of <strong>paper <\/strong>without a filler is white because <strong>light is scattered from the individual cellulose fibers<\/strong>, and the <strong>foamy head on a glass of beer is white for the same reason<\/strong>; there are so many scattering surfaces on the thin bubbles that the light rays <strong>never traverse a sufficient thickness of the fluid to show the yellow color.<\/strong> In fact, body-color pigments lose their color if sufficiently finely ground, but surface-color pigments do not do so. &#8211; p.239<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Interference and diffraction<\/h3>\n\n\n\n<p>The Heiligenschein, a bright region seen around the head of our shadow on a dew-covered lawn, is not a corona but merely the reflection of the sun&#8217;s light over a limited range of angles in the water drops on the grass and does not contain color. &#8211; p.269<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Colorants of many types<\/h3>\n\n\n\n<p>It is necessary to distinguish between dyes and pigments. As a good first approximation, it can be said that <strong>most dyes are soluble<\/strong>, whereas <strong>all pigments are insoluble<\/strong> in the medium under consideration and usually <strong>require a binder<\/strong> to hold them to the substrate. Yet some insoluble dyes, such as the blue indigo, are applied to a fabric in soluble colorless form and are subsequently converted into the insoluble colored form. Disperse dyes are insoluble substances applied to a fiber in such a finely divided form as to be the equivalent of a solution. &#8211; p.281<\/p>\n\n\n\n<p>Several areas of colorants do not fall into the pigment-dye division; thus neither designation is appropriate for substances used to color glasses, enamels, and glazes. Also not included in the pigment-dye classification are the allochromatic transition metal-caused colorations in inorganic substances, minerals, and gemstones. &#8211; p.283<\/p>\n\n\n\n<p>The tail of an absorption band in the ultraviolet absorbs a little violet to give a yellow color, but at a stronger absorption intensity, violet, blue, and blue-green are absorbed to give an orange color. At a high enough absorption all visible light could be absorbed if the absorption tail extends across the entire visible spectrum. As another example, consider a substance with strong absorptions in the red and blue green, a medium absorption in the yellow region, and essentially zero absorption in the violet. At a low concentration, the higher sensitivity of the eye for yellow than for violet will result in the perception of a yellow color. At a high concentration, only a negligible amount of yellow is transmitted, and the color produced by the violet transmission band will now be perceived. &#8211; p.284<\/p>\n\n\n\n<p>Pigments are used in paint-type compositions in two often complementary ways: for coloration and for opacity. When compared to the refractive index of the vehicle, the paint medium itself, the refractive index of the pigment is significant, as is the pigment particle size. For the best opacity or hiding power, so that a thin paint film can cover and hide different colors beneath it, it would be desirable to have good light scattering and reflection at the pigment surface. This results from a large refractive index difference. Most white pigments used in paints have refractive indices well above 2.0 (e.g., anatase, rutile, lead white, zinc white) compared to the 1.5 of a typical linseed oil paint. Although just as &#8220;white,&#8221; chalk with a refractive index of 1.6 would scatter very poorly in such a paint and would have only weak hiding power. When used with water and a little glue, however, in the form of whitewash, the difference between the 1.6 refractive index of the dried chalk and the 1.0 of air gives a satisfactory hiding power. &#8211; p.285<\/p>\n\n\n\n<p>The <strong>very high refractive index of intensely colored inorganic pigments<\/strong>, such as cadmium yellow or cobalt blue, reflects light very effectively, and these pigments are therefore<strong> used in larger particles to avoid scattering, typically 1 to 10 microns and up<\/strong>. Too fine grinding for such pigments produces a loss of color or desaturation. In the case of the organic pigments with a <strong>refractive index approximately matching that of the vehicle<\/strong>, scattering is not important, and the absorption power or <strong>strength of the color increases as the particles are made smaller.<\/strong> &#8211; p.285<\/p>\n\n\n\n<p>Commercial paints, including artists&#8217; paints, are carefully made of pigment particles ground and separated to be of optimum size. This is one way of identifying old paintings made at a time when artists ground their own paints by hand, with pigments containing a wide range of particle sizes. &#8211; p.285<\/p>\n\n\n\n<p>There are, of course, the dyes used on yarns and fabrics, ranging from the oldest cotton, wool, and silk-to the wide range of synthetic polymers. Next there are dyes used in plastics (note that pigments may also be incorporated into plastic, however with a reduction of transparency), paper, leather, fur, wood, and wax. Finally, there are the dyes used for food, drugs, and cosmetics, including hair, soap, toothpaste, and the like, controlled because of their potential hazards to health. Materials may be dyed uniformly, or processes such as printing, batik dyeing (the process of blocking off parts of a fabric with wax to prevent dye absorption), and tie dyeing (tying or knotting a fabric to control the dye absorption) may be used to produce patterns. &#8211; p.291<\/p>\n\n\n\n<p><strong>Glasses are disordered systems formed by cooling a melt sufficiently rapidly so that crystallization does not have time to occur.<\/strong> The highly irregular structure of the melt is, accordingly, frozen into the glassy or vitreous state. As a result, the <strong>ligand field concepts cannot be expected to apply rigorously<\/strong>, since a given type of colorant atom may have a wide range of environments around it; the energy levels will become broadened and lead to<strong> very wide absorption bands.<\/strong> &#8211; p.302<\/p>\n\n\n\n<p>Glazes are opaque, low-melting glasses that are used as a protective nonabsorbent layer on the surface of ceramics and may also be used to decorate ceramic and glass objects. Enamels are similar transparent or opaque low-melting glasses used to protect and\/or decorate the surface of metals. <strong>The coloration of glazes and enamels is thus the same as that of glasses. <\/strong>&#8211; p.304<\/p>\n\n\n\n<p><strong>Filters that transmit or block a part of the spectrum selectively can be based on one of four mechanisms: absorption, interference, birefringence, or scattering.<\/strong> We can distinguish band-pass filters and band-blocking and band reflection filters, which transmit and block, respectively, a section of the spectrum and which may be narrow or wide. Then there are sharp and broad cutting filters, which may be short-wave-pass filters, that is, passing the short wavelengths at the blue end of the spectrum but blocking the longer wavelengths, and long-wave-pass filters, where the reverse applies. &#8211; p.308<\/p>\n\n\n\n<p>Many of the imputed biological diffraction grating colors have been shown on detailed study to originate in thin-film interference; observation of a periodic grating structure is quite insufficient as evidence. As reported by Mason, there is a diffraction grating present on some butterfly wing scales, and its effects can be observed under the microscope; the naked eye, however, does not detect these diffraction colors but instead sees thin-film interference to which the usually observed color is properly attributed. &#8211; p.318<\/p>\n\n\n\n<p>When photosynthesis stops because of the absence of water, nutrients, light, or as a result of low temperatures or short daylight hours in the fall of the year, the <strong>green chlorophyll-produced color fades<\/strong>. The resulting color depends on the genetically determined presence of other pigments:<strong> yellow or orange from carotenes, reds or purples from anthocyanins, browns from tannins, or combinations of these.<\/strong> &#8211; p.325<\/p>\n","protected":false},"excerpt":{"rendered":"<p>by Kurt Nassau Some fundamentals: color, light and interactions The three subtractive primary colors &#8211; magenta, yellow, and cyan &#8211; are complementary to the three additive primary colors. If we remove the red-orange region from the full spectrum, we obtain cyan. Removing the violet-blue region gives yellow, and removing green gives magenta. To predict subtractive &#8230; <a title=\"The Physics and Chemistry of Color\" class=\"read-more\" href=\"https:\/\/www.labri.fr\/perso\/barla\/blog\/?p=39825\" aria-label=\"Read more about The Physics and Chemistry of Color\">Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[621],"tags":[],"class_list":["post-39825","post","type-post","status-publish","format-standard","hentry","category-books"],"_links":{"self":[{"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/posts\/39825","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=39825"}],"version-history":[{"count":17,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/posts\/39825\/revisions"}],"predecessor-version":[{"id":40355,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/posts\/39825\/revisions\/40355"}],"wp:attachment":[{"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=39825"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=39825"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=39825"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}