MOLECULAR ENERGY 301 



medium to transmit a wave train without absorption 

 when the natural frequency of its oscillators is the same 

 as that of the waves. Those oscillators of a medium 

 which are nearest the source of the radiation absorb 

 the most and those farthest away the least of the energy. 

 For this reason a thin sheet of black paper may prove 

 insufficient to protect photographic films in strong sun- 

 light when a sheet of double the thickness may do so. 



The oscillators of a medium which are absorbing 

 radiation are vibrating with the increased amplitudes. 

 Now, it makes no difference how they were set into 

 vibration, they will also radiate energy. (It is im- 

 material, for example, whether a piano string is set 

 into vibration by a wave train or by a blow on the 

 proper key, it will send out a wave tram of its own 

 frequency.) The absorbing oscillators, at and near 

 the surface, thus act as radiators. 1 The frequency of 

 the radiation, which is " reflected," must then be that 

 which is absorbed. Reflection is really " re-radiation." 



In the other method for obtaining the natural fre- 

 quencies it is assumed that at the melting point of a 

 solid the amplitude of vibration of the atoms is prac- 

 tically equal to the average distance between the centers 

 of adjacent molecules. At the melting point of all 



1 The reason that a mirror surface must be plane and smoothly 

 polished is now apparent. It must present to the impressed radi- 

 ation a smooth front of oscillators so that the energy which they 

 radiate may all be available at some distant point. If the surface 

 is irregular the energy radiated by some of the oscillators will render 

 unavailable at the desired distant point some of the energy radiated 

 by the other oscillators. Or, as it is more usually said, there will 

 be a scattering and an interference of the reflected waves. Scatter- 

 ing is negligible if the irregularities are small as compared to the 

 wave length of the incident radiation. 



