INTERFEKOIVIETRIC METHODS 



when traveling through transparent media. 

 While absolute measurements of the velocity 

 of light are difhcult, evaluation of relative 

 variations is relatively easy.- The instru- 

 ments used for this purpose are generally 

 called interferometers. 



Today, radiant energy is considered as an 

 electromagnetic perturbation the two fields 

 of which are normal to one another and also 

 normal to the direction of propagation of the 

 perturbation: the vibrational field is trans- 

 versal. A monochromatic radiation is char- 

 acterized by its 'period, T, or time in seconds 

 required by one energy wave to effect a com- 

 plete oscillation, or in Maxwell's theory for 

 the corresponding line of force h to effect a 

 complete rotation around the point 0. 



During this time T, the perturbation 

 travels in a direction x with the velocity C 

 (light velocity in a vacuum). The distance 

 traveled is: 



C-T = X 



m 



this being called the wavelength, measured in 

 Angstrom units.* 



The period T is not directly 

 Only in a vacuum, C being the 

 radiations, is X directly propor 

 The frequency, v, of a radiation 

 ber of oscillations contained in 

 CT over a period of one second 



measurable, 

 same for all 

 tional to T. 

 is the num- 

 the distance 



V = C/\ 



US) 



Frequencies are expressed in Fresnel units 

 (10~^2). The wave numbers, v, are more con- 

 veniently handled than either X or v. They 

 are the number of vibrations contained in 1 



cm: 



1 _ l.lQs cm 

 X ~ X( in A) 



(cm-i) 



(M) 



The quantities C and X vary with the me- 

 dium traversed, but v (and T) do not. The 

 latter quantities characterize the conditions 

 under which the radiation is generated ( ex- 

 cited orbital, atomic number, temperature, 



* See equations 1-11 in two preceding articles. 



etc.). The quantity C (or X), on the contrary, 

 characterizes the conditions under which the 

 radiation travels. 



The value of C can be calculated in an- 

 other way, by means of the classical dimen- 

 sional equations. 



Both Coulombic electrostatic and mag- 

 netic attraction forces between two unit 

 charges e can be measured. For the electric 

 attraction one has: 



F = ± 



1 ^ 



or 



or in dimensional form: 



[el = Ko^'Hl^i^Ui^T-^ 



(15) 



(16) 



Since a moving charge e generates a mag- 

 netic field, the latter may also be a measure 

 of e: 



[e] = [Mi/2Li/Vo-^'2] (17) 



(since force = mass X acceleration = 

 MLT-^, and d^ = U). The two above quanti- 

 ties are equal: 



[M^i2.L^i2.^-ii2] = [Ko^im^i^L^i^T-^] (18) 



or 



[LT-^[m-"^-K,- 



1/2 1 = 



Va'o- 



= V 



(19) 



MO 



Hence, the quantity V found has the di- 

 mension of a velocity whose value is equal 

 to the ratio of the e s u and emu units. It 

 was experimentally found equal to the ve- 

 locity of hght ill vacuo (Weber, 1856) : C = 

 2.9986 X IQio cm/sec. 



The value of C calculated above repre- 

 sents a maximum. It is not the only charac- 

 teristic velocity of electromagnetic phe- 

 nomena which needs to be considered. The 

 group-velocities of Rayleigh are useful in 

 interpreting the peculiarities of refractive dis- 

 persion within an absorption hand, even when 

 "monochromatic light" is used, for even the 

 finest spectral lines have always a finite 



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