88 



ELECTROMAGNETIC RADIATIONS AND MATTER 



yellowish to clear. In the dark, in vivo, the color is restored. The process can 

 be summarized as follows: 



photons + rhodopsin 



k 



A, 



(bleaching) 

 k 2 



bleached vitamin A -f energy 



(to nerve endings) 



+ 



retinene 



+ energy 

 (regeneration) 



The scheme above indicates that the greater the intensity of the incoming 

 light, the more will the rhodopsin be bleached. In twilight most of the pig- 

 ment exists as rhodopsin, and the sensitivity is greatest. In daylight, most 

 of it will be bleached, and the sensitivity least. "Dark-adaptation" is very 

 familiar to us all; it is slow because the speed of regeneration of rhodopsin is 



o 



in 



z 



UJ 

 Q 



< 



a. 

 o 



0.7 - 



0.6 - 



0.5 - 



0.4 - 



0.3 - 



0.2 - 



0. I - 



250 300 350 400 450 500 550 

 3000 4000 5000 



600 m M 



o 

 6000 A 



WAVELENGTH 



Figure 4-9. The spectrum of human scotopic (twilight) vision sensitivity 

 (crosses), and the absorption spectrum of rhodopsin (solid curve) are 

 the same. (After Rushton. 1 ) 



