Contemporary Advances in Physics, XVI 



The Classical Theory of Light, Second Part^ 



By KARL K. DARROW 



MEASUREMENT of wave-lengths is the subject which we shall 

 now consider. So entitled, the topic seems unpromising, as 

 some dry exercise in mensuration ; but in truth it is distinguished for 

 beauty and variety, and implicated with the whole of modern physics. 

 This is not measurement of the lengths of palpable objects, as pieces 

 of lumber or cloth, which are laid alongside of a yardstick or clamped 

 in the jaws of a gauge. In optics, the methods of measuring wave- 

 lengths are the methods of proving that waves exist, therefore of 

 testing the undulatory theory of light. One could not reasonably 

 ask for evidence of light-waves more convincing than the concord of 

 the values obtained for the wave-length say of sodium yellow light, 

 by all the diverse instruments which act by causing interference or 

 diffraction: Newton's tapering film of air between a lens and a plate, 

 Fraunhofer's grid of iron wires, the tilted mirrors of Fresnel, Michelson's 

 echelon, and all the many gratings and interferometers continually in 

 use in laboratories and classrooms. Wave-lengths of X-rays are com- 

 puted from the diffraction-patterns imposed on X-ray beams by 

 intercepting crystals, and these patterns were the evidence which 

 showed some fifteen years ago that the rays are of the nature of 

 undulations, though it could not disprove that in some paradoxical 

 way they are also of the nature of corpuscles. From similar diffraction- 

 patterns imposed by crystals on electron-streams it follows that these 

 also are partly of the nature of waves, and again the patterns have 

 supplied the values of the wave-lengths. 



Moreover, evidence for waves and values of their lengths are 

 only part of what a grating can supply. Once we are sure that we 

 know the wave-length of a certain kind of light, we can send it against 

 a grating and study the diffraction-pattern with the opposite intent: 

 analyzing not the light but the grating, and deducing the widths and 

 the spacings of the slits, if it is an alternation of slits and stops — the 

 spacing and the shaping of its grooves, if it is an engraving on metal 

 or glass — the arrangement of the atoms and of the electricity within 

 the atoms, if it is a crystal. Therefore the methods for measuring 

 wave-lengths of X-rays are also those for exploring the structures of 

 solids and of the atoms of which these are composed. Remember 



1 Continued from the April, 1928, issue. 



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