= Propagating angular-spectrum component
A \b[<propagating angular-spectrum component>] is a transverse <Fourier transform> component whose longitudinal <wavenumber> is real in a homogeneous lossless medium. With $q^2=k_x^2+k_y^2<k^2$ and the forward <outgoing branch of the angular spectrum>,
$$
k_z=\sqrt{k^2-q^2}=k\cos\theta,\qquad q=k\sin\theta.
$$
Its propagation factor $e^{ik_zz}$ has unit magnitude and a distance-dependent <wave phase>.<Interactive Optics by Sander Konijnenberg, Aurèle J.L. Adam and H. Paul Urbach>[Scalar Diffraction Optics, Angular Spectrum Method, propagating components and longitudinal wavenumber]{ref}
For an original component with $k_x=0.6k$ and $k_y=0$, $k_z=0.8k$ and $\theta\simeq36.87\,{}^\circ$. It repeats its phase along $z$ over distance $2\pi/k_z=1.25\lambda$, even though its wavelength measured along its propagation direction remains $\lambda$. The axial phase period is not an energy-transit time.
Preserving each component's magnitude does not preserve the reconstructed spatial intensity. Different values of $k_z$ accumulate different phases; their coherent superposition changes the <interference> pattern and can spread a beam. This is the angular-spectrum description of <diffraction>.
Energy crossing a reference plane also depends on direction. For a vacuum <plane electromagnetic wave> with electric-field magnitude $E_0$, the time-averaged <Poynting vector> has magnitude $|E_0|^2/(2Z_0)$ along the propagation direction. Its normal component through a plane of constant $z$ is that magnitude times $\cos\theta$, where $Z_0$ is the <vacuum impedance>. Equal electric-field magnitudes at angles zero and $60\,{}^\circ$ therefore give normal fluxes in the ratio $1:1/2$.
The boundary case $q=k$ is grazing, with $k_z=0$ and no forward normal phase advance. For $q>k$, the root becomes imaginary and the component is instead an <evanescent angular-spectrum component>. These classifications apply to the stated real-wavenumber, lossless model; material absorption requires a consistent complex-wavevector treatment.
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