Absorption and quantum theory

Absorption in the visible region, with which dyes and pigments are concerned, is electronic in origin. Quantum theory states that energy transformations on an atomic or molecular scale occur discontinuously, in discrete packets or quanta. 

V = frequency of absorbed radiation 2. = wavelength of absorbed radiation h = Planck s constant c = velocity of light. 

In general, colourless organic substances absorb in the ultraviolet region where /l 400nm and AE is large. For ethylene (CH2=CH2), AE is approximately 710kJ/mole and absorption occurs in the UV at c. 175 nm. 

In longer chains having alternate single and double bonds =CH—CH= CH— (conjugation), the principal absorption peak is displaced towards longer wavelengths. 

In lycopene, the chief colouring matter of the marigold, thirteen double bonds are present in the conjugated chain, A ax moves from the UV to the blue region of the visible spectrum and the colour seen is a reddish yellow. 

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