396 PROTOPLASM 



He defended the view that Hght consists of streams of minute 

 particles. He beUeved this because Hght travels in a straight 

 line which is characteristic of moving bodies. His contemporary, 

 the Dutch physicist Huygens, advocated the wave theory (1678) 

 and introduced the concept of an all-pervading ether to explain 

 the transmission of the waves through space. So great was 

 Newton's influence that the only prominent men of the eighteenth 

 century who supported the undulatory theory were Leonard 

 Euler and Benjamin Franklin. But during the next century, 

 things were reversed, and the corpuscular theory of Newton fell 

 into ill repute — so much so that, fifty years ago, S. P. Langley 

 wrote of it as ''pernicious," for, if light is material, then "radiant 

 heat, if affiliated to light, must be regarded as material too." 

 And now, in the twentieth century, both theories are accepted. 



The first break from the wave theory of light was initiated by 

 the German physicist Planck, who stated that when a body 

 becomes red hot, it does not give off energy continuously, as the 

 wave theory would demand, but radiates heat in discontinuous 

 units, bursts, or quanta of energy. Einstein then called attention 

 to the fact that Planck's conclusion would be satisfied by the 

 view that radiation is not wavelike, but is of matter, that is to 

 say, consists of particles. Thus arose the quantum theory which 

 in certain features resembles Newton's corpuscular hypothesis. 

 The particles, or quanta, of light are now known as photons. 

 The energy value of a quantum is the product of the wave fre- 

 quency V and the constant of Planck h. Expressed in this way, 

 the energy of a quantum of green light corresponds to 2.5 electron 

 volts. On the same basis, the energy given off when four 

 hydrogen atoms unite to form one helium atom is 27,000,000 

 volts. 



The diffraction of light, and of X rays by crystals, is convincing 

 evidence that light and X rays consist of rays. The photo- 

 electric effect is just as convincing evidence that they consist 

 of particles. If light falls upon metals such as zinc or sodium, a 

 current of negative electricity — a stream of electrons— escapes 

 from the metallic surface. This is the photoelectric effect. It 

 is more prominent in the case of X rays, which have the capacity 

 to eject electrons from many kinds of surfaces. Physicists saw 

 the futility of trying to explain the photoelectric effect on the 

 basis of waves. Only particles — photons — can account for it. 



