Herein, we make the substitution according to (10), than we have
$(W)=A\sin {\frac {2\pi }{T}}\left(1{\frac {\varkappa }{\omega }}\right)\left(t{\frac {x}{\omega }}\right).$
This gives for x = ϰt:
$({\overline {W}})=A\sin {\frac {2\pi t}{T}}\left(1{\frac {\varkappa ^{2}}{\omega ^{2}}}\right)=A\sin {\frac {2\pi t}{T'}}{,}$

14')

thus we have an illuminating plane (moving parallel to the Xaxes), which oscillates with a wavelength $\textstyle {T'=T/\left(1{\frac {\varkappa ^{2}}{\omega ^{2}}}\right)}$ (only different of the second order of T). The propagated oscillation can be written:
$(W)=A\sin {\frac {2\pi }{T'\left(1{\frac {\varkappa }{\omega }}\right)}}\left(t{\frac {x}{\omega }}\right).$

14)

Thus we get, within the propagated wave, a reduced period of oscillation in the relation of $\left(1{\frac {\varkappa }{\omega }}\right)/1$.
Is the observer is in motion as well, then:
$(W')=A\sin {\frac {2\pi }{T'\left(1{\frac {\varkappa }{\omega }}\right)}}\left(t{\frac {x'+\varkappa 't}{\omega }}\right)$
$=A\sin 2\pi \left(t{\frac {(\omega +\varkappa '}{T'(\omega \varkappa )}}{\frac {x'}{T'(\omega \varkappa )}}\right).$
This formula gives the principle of Doppler for plane waves. But it is in no way universal, but essentially presupposes a plane wave with constant amplitude throughout.
2) The same plane is to be set in oscillation by the law:
${\overline {W}}=Ae^{(\mu y+\nu z){\frac {2\pi }{T\omega }}}\sin {\frac {2\pi t}{T}}$
 as it similar occurs when a wave with initially constant amplitude travels through a prism of an absorbing substance  then for the propagated wave it is given:
$W=Ae^{\frac {2\pi (\mu y+\nu z)}{T\omega }}\sin {\frac {2\pi }{T}}\left(t{\frac {x\sigma }{\omega }}\right){\text{ where }}\sigma ={\sqrt {1+\mu ^{2}+\nu ^{2}}}.$