Any coloured compound is not a Dye or Dyestuff. A dye is acoloured organic compound that absorbs light strongly in the visible region andcan firmly attach to the fiber by virtue of chemical and physical bondingbetween group of the dye and group on the fiber. To be of commercial importancea dye should be fast to light, rubbing and water.

Colour and dye have always played an important role in thelife of man from time immemorial. Preparation of a colour and dyeing of clothdate back to antiquity. Fabrics dyed in indigo were found in the tombs ofpredyanstic Egypt. Let us now try to understand how we get sensation of colour.

Modern theory of colour:

Colour is a physiological sensation associated with thewavelength of light striking the retina of the eye. The sensation of colour isproduced when light having a wavelength within the visible region ofelectromagnetic spectrum strikes the retina of the eye.

The visible region of the spectrum extends from 4000 to 7500 in wavelength.

When white light falls on a substance, the light may becompletely reflected and in this case substance will appear white. If it iscompletely absorbed, the substance will appear black. If a substance absorbsall visible light except that corresponding to e.g. yellow, it willtransmit or reflect only yellow colour and will be seen as yellow.However, it is generally seen that, light of only one colour is absorbed inwhich case the substance will appear to have the complementary colour. Thus, ifthe light is absorbed from the violet region of spectrum, the substance will beseen as yellow. If light is absorbed from the red region, the substance willappear green.

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About the Author

The authoris associated with Pakistan Council of Scientific & Industrial Research(PCSIR) Lab complex, Karachi as a Senior Research Associate.