418 HAROLD F. BLUM 



A + hv = A* (1) 



where ^ is a molecule of a given substance that will be referred to as 

 the light absorber, A* is the same molecule after it has been raised 

 to an excited state by the capture of a quantum of radiant energy hv, 

 h is Planck's constant (6.6236 X 10~" erg sec), and v is the frequency 

 of the radiation. The frequency is related to the wavelength X as 

 follows : 



V = c/\ (2) 



where c is the velocity of light (3 X 10^" cm. sec.~0- The following 

 calculation, made for the wavelength 500 mju (= 5.0 X 10 ~^ cm.), 

 shows that the quantum has the dimensions of energy and indicates 

 its magnitude : 



, //AN rroQrv/in-27 3 X 10^" cm. sec."^ 



hv = h{c/\) = 6.6236 X 10^^ erg sec. 



5.0 X 10~^ cm. 



= 3.97 X 10-12 erg = 2.48 electron volts 



It is noted that the quanta dealt with in photochemical processes are 

 much smaller than those of the ionizing radiation, which has been 

 treated in other chapters. Most photobiological processes involve the 

 action of many quanta, but this is not universally true, as illustrated 

 in the values collected in Table I. When, as in scotopic vision of the 

 human eye, only a few quanta are required to ehcit a response, one 

 must be very careful indeed about stray light; whereas in other 

 cases one can be less exacting. Also, it is noted that in destructive 

 processes, such as damage to cells by ultraviolet radiation, the cell 

 may absorb many quanta of radiation without being severely dam- 

 aged. 



The primary reaction (1) is the initial event in any photochemical 

 process, that is, a molecule must absorb a light quantum in order to 

 reach the excited state from which it may proceed to take part in a 

 chemical reaction. The reaction may vary from direct splitting of 

 the light absorber molecule to long chain reactions (4,9). A treat- 

 ment of the reactions that may follow the primary event is beyond 

 the scope of this chapter, and for the moment it is only important to 

 note that photochemical reactions of any kind depend upon the ab- 

 sorption of radiant energy, a fact often referred to as the first law of 

 photochemistry, or sometimes, the Grotthus-Draper law. 



