INTERMITTENCY FACTOR IN FLASHING LIGHT 



1449 



For low integrated light intensities, i. e., low values of the product 

 (energy of a single flash X number of flashes per unit time), the factor iet 

 must be unity (since in this case, all the absorbed light quanta can be 

 utilized for photosynthesis, with the highest possible quantum yield, 

 whether they are supplied continuously or in flashes). The factor t^t will 

 decline below unity when the flash energy becomes so high that more 

 intermediates are produced in a single flash than the available catalyst 

 Eb can handle in one batch (since, according to our assumptions, the frac- 

 tion of the intermediates that finds no free catalyst is lost by back reac- 

 tions). Thus, if one plots the rate of photosynthesis in flashing light with 



2 3 4 5 6 



AVERAGE INTENSITY, f 



Fig. 34.6. Light curves for flashing light (after Weller and Franck 1941 ). The number 

 of light flashes per second, 7, is the parameter in this set of curves. 



different frequencies of flashes against the integrated intensity (or, what is 

 essentially the same, the average incident energy per unit time), one expects 

 to obtain a picture of the type of figure 34.6, based on actual experimental 

 results of Weller and Franck (1941). In this figure, the ratjo of yields in 

 flashing fight and in continuous light with the same value of /, is the factor 

 Iei. The figure confirms that this factor is unity for low values of /. 

 It declines below unity, first for widely spaced flashes (where, in order to 

 achieve the same average intensity of illumination, one has to use stronger 

 flashes), and later for flashes that are more closely spaced. When the dark 

 intervals, ta, approach zero, i. e., when the flash frequency becomes very 

 high, the light curve for flashing light must become identical with that for 

 continuous light. 



Figure 34.6 confirms that the yield in flashing light never exceeds that in 

 continuous fight of the same integrated intensity; i. e., that the factor ist 

 never is larger than unity. A theoretical proof of this rule was given in 

 part A for alternating light; the same proof can easily be generalized to 

 include intermittent light with any ratio of t* and ta. 



This proof is illustrated by figure 34.7. The shaded areas represent 



