10 BIOLOGICAL EFFECTS OF RADIATION 



other way, he means either that his apparatus has been cahbrated against 

 some diffraction apparatus, or that he has measured the energy of the 

 photons and has computed X from them by equation {2). 



Measurements of wave-length in the visible and ultra-violet ranges of 

 the spectrum are generally made upon the diffraction patterns produced 

 by diffraction gratings which are metal plates ruled with parallel grooves 

 numbering several thousands to the centimeter. When monochromatic 

 light falls through a slit against such a grating, the pattern is of such 

 a nature that a photographic plate set at an adequate distance receives a 

 sequence of linear imprints (images of the slit) of which the position of 

 any one suffices to show the wave-length of the radiation; hence the term 

 "line" for monochromatic light. Gratings for use in the infra-red are 

 sometimes made of parallel wires. Various other instruments producing 

 diffraction patterns, such as the echelon and diverse types of interferome- 

 ter, are sometimes used in these regions of the spectrum; their patterns 

 often bear no resemblance to the sharp lines formed by a grating. For 

 X-rays the use of ruled gratings is becoming extensive, though it is not 

 Ukely to supersede the original method, i.e., that of using crystals of which 

 the regular alignments of atoms serve as the rulings of a natural three- 

 dimensional grating. In the gamma-ray region it is necessary to measure 

 the energy of the photons and deduce X from it by equation {2) . 



EXTERNAL PHOTOELECTRIC EFFECT OF METALS, AND MEASUREMENT 



OF PHOTON ENERGY 



Having spoken of the method of measuring wave-length of light, I 

 shall now speak of the principal method for measuring energy of photons, 

 though this procedure requires me to describe out of its due order one of 

 the most important phenomena of the interaction of light and electricity : 

 the external photoelectric effect of metals, often designated simply as 

 "the photoelectric effect." 



Let a beam of monochromatic light of wave-length X be sent against 

 a metal target enclosed in an evacuated tube, the target being connected 

 to a wire passing through the tube wall so that it becomes an electrode, 

 and the tube being fitted with other electrodes and metallic grids or 

 gauzes connected to wires and so arranged that electric fields can be 

 applied to ihe target. If X is greater than a certain "threshold wave- 

 length" depending on the target metal, nothing happens. If X is less 

 than this threshold wave-length, electrons emerge from the metal. If 

 the target and the other electrodes are charged to suitably chosen relative 

 potentials, the electric field will draw these emerging electrons to some 

 other electrode, and so long as the light continues to shine steadily, there 

 will be a steady photoelectric current from the target. It will be suffi- 

 cient for our purpose to visualize the simplest possible arrangement: 



