THE VISUAL PIGMENTS 



Optical densities are additive. Thus if a solution of density D^ is 

 juxtaposed to a solution of density Dg, the combination has a density 

 of Di + Da- Similarly the density of a mixture of solutes can be 

 regarded as the sum of the densities of each solute respectively, 

 provided chemical interaction has not occurred. 



100 

 90 



St 

 (0 



% 50 



en 



2 40 



c 



V 



o30 



Q. 



20 



10 



0-5 

 Optical density (log 



1-5 



Fig. 1.8. Relation between optical density, Z), and fraction of light 

 absorbed, hhslhnc The dashed line is drawn according to the equation, 



/aW/inc = 2-303Z). 



The relation between the optical density, D;, and the fraction of 

 light absorbed, /abs//inc, can be obtained as follows, 



Dx = logio /inc//trans 



i-e. 10^^ = /inc//trans 



whence /abs//inc = (Zinc - /trans)//inc = 1 - lO"^'^ 



This relationship is shown graphically in Fig. 8 by the full-line curve. 

 The expression 1 — 10-^^ for the fraction of hght absorbed can be 

 expanded into the infinite series 



|2 



18 



