Theory of Colour

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THE THEORY OF DYEING Assignment By Professor : Yasir Ansari Name : Ahmer Adnan Baloch Colour Perception We see colour with the sensors in the retina of the eye called rods and cones. The rods are sensitive to low light and the cones, which require a greater intensity of light, are sensitive to colour. The message is passed to the optic nerve and then on to the brain. We see colour because of the Rods and Cones in our eyes: There are about 12million rods and about 6 to 7 million cones, in the
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  THE THEORY OF DYEING Assignment By Professor : Yasir  Ansari Name:Ahmer Adnan Baloch  Colour Perception We see colour with the sensors in the retina of the eye called rods andcones. The rods are sensitive to low light and the cones, which require a greater intensity of light, are sensitive to colour. The message is passed to the optic nerveand then on to the brain.We see colour because of the Rods and Cones in our eyes:There are about 12million rods and about 6 to 7 million cones, in the human eye.Rods are more sensitive than the cones but they are not sensitive tocolour, they perceive images as black, white and different shades of grey. Morethan one thousand times as sensitive, the rods respond better to blue but verylittle to red light.Each cone contains one of three pigments sensitive to either RED GREENor BLUE.Each pigment absorbs a particular wavelength of colour. There are shortwavelength cones that absorb blue light, middle wavelength cones that absorbgreen light, and long wavelength cones that absorb red light.When we observe a colour that has a wavelength between that of theprimary colours red, green and blue, combinations of the cones are stimulated. Anexample could be that yellow light stimulates cones that are sensitive to red andto green light. The result is that we can detect light of all colours in the visiblespectrum.People who suffer colour blindness have less numbers of particular conesthan normal, so they get colours confused.If we lose our eye sight, the body adapts and receives colour rays throughthe skin. It takes time for the body to adapt, but it has been shown that peoplewho are blind, can differentiate between different colours.The eye picks up colour and light by the Rods and Cones. It is the Conesthat detect Colour. Each cone contains one of three pigments sensitive to either RED GREEN or BLUE.DyeA dye can generally be described as a colored substance that has anaffinity to the substrate to which it is being applied. The dye is generally applied inan aqueous solution, and may require a mordant to improve the fastness of thedye on the fiber.  Auxochrome  This is a group of atoms attached to a chromophorewhich modifies the ability of that chromophore to absorb light. Example-OH , - NH2, Aldehydes.An auxochrome is a functional group of atoms with nonbonded electrons which,when attached to a chromophore, alters both the wavelength and intensity of absorption. If these groups are in direct conjugation with the pi-system of thechromophore, they may increase the wavelength at which the light is absorbedand as a result intensify the absorption. A feature of these auxochromes is thepresence of at least one lone pair of electrons which can be viewed as extendingthe conjugated system by resonance.It increases the color of any organic compound. For example, benzene does notdisplay color as it does not have any chromophore but nitrobenzene is pale yellowcolor because of the presence of nitro group. Para-hydroxynitrobenzene exhibitsa deep yellow color. Here an auxochrome (-OH) is conjugated with thechromophore -NO2. Similar behavior is happens in azo benzene (red color) butpara-hydroxy azobenzene is dark red color.There are mainly two types of auxochromes-1. Acidic -COOH, -OH, -SO3H2. Basic -NHR, -NR2, -NH2The presence of an auxochrome in the chromogen molecule is essential to make adye. However, if an auxochrome is present in the meta position to thechromophore, it does not affect the colour.An auxochrome is known as a compound that produces red shift as it increasesthe wavelength of absorption therefore moving closer to infrared light.  Chromophore A chromophore is part (or moiety) of a molecule responsible for its color.When a molecule absorbs certain wavelengths of visible light and transmits or reflects others, the molecule has a color. A chromophore is a region in a moleculewhere the energy difference between two different molecular orbitals falls withinthe range of the visible spectrum. Visible light that hits the chromophore can thusbe absorbed by exciting an electron from its ground state into an excited state.In biological molecules that serve to capture or detect light energy, thechromophore is the moiety that causes a conformational change of the moleculewhen hit by light.Chromophores almost always arise in one of two formsconjugated pi systems or resonating systems. and metal complexes.[contradiction]In the former, the energy levels that the electrons jump between are extended piorbitals created by a series of alternating single and double bonds, often inaromatic systems. Common examples include retinal (used in the eye to detectlight), various food colorings, fabric dyes (azo compounds), lycopene, -carotene,and anthocyanins.The metal complex chromophores arise from the splitting of d-orbitals by bindingof a transition metal to ligands. Examples of such chromophores can be seen inchlorophyll (used by plants for photosynthesis), hemoglobin, hemocyanin, andcolorful minerals such as malachite and amethyst.A common motif in biochemistry is chromophores consisting of four pyrrolerings. These come in two types:* the pyrroles form an open chain, no metalphytochrome, phycobilin, bilirubin* the pyrroles form a ring (porphyrin), with a metal in the centerheme, chlorophyll Color abstractions The foundations of pre-20th-century color theory were built around pure or idealcolors, characterized by sensory experiences rather than attributes of the physicalworld. This has led to a number of inaccuracies in traditional color theoryprinciples that are not always remedied in modern formulations.[citation needed]The most important problem has been a confusion between the behavior of lightmixtures, called additive color, and the behavior of paint or ink or dye or pigmentmixtures, called subtractive color. This problem arises because the absorption of light by material substances follows different rules from the perception of light bythe eye.  
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