(67)

where ${\xi}^{*}$ is a real number. To understand the meaning of Eq. (67), let us substitute it into the general expression Eq. (34); we get

${u}_{z}(x,y,t)=h(y){e}^{i(\xi x-\omega t)}=h(y){e}^{i(i{\xi}^{*}x)}{e}^{-i\omega t}$ (68)

(68)

i.e.,

${u}_{z}(x,y,t)=\left[h(y){e}^{-{\xi}^{*}x}\right]{e}^{-i\omega t}$ (69)

(69)

Equation (69) describes an *evanescent*^{1} *wave*, which has a decaying amplitude and only exists close to the excitation ...

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