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

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

Modern theory of colour:

Colour is a physiological sensation associated with the wavelength of light striking the retina of the eye. The sensation of colour is produced when light having a wavelength within the visible region of electromagnetic 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 be completely reflected and in this case substance will appear white. If it is completely absorbed, the substance will appear black. If a substance absorbs all visible light except that corresponding to e.g. yellow, it will transmit or reflect only yellow colour and will be seen as yellow. However, it is generally seen that, light of only one colour is absorbed in which case the substance will appear to have the complementary colour. Thus, if the light is absorbed from the violet region of spectrum, the substance will be seen as yellow. If light is absorbed from the red region, the substance will appear green.

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About the Author

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