Optical Effects
A general principle concerning plane waves states you cannot see the wave train enter the station, but you can see it leave. That is, if you look through oncoming wavefronts at distant objects, you will see no optical distortion, but if you turn and look through departing wavefronts at distanct object, you will see optical distortions. Specifically, the null geodesic congruence generated by the null vector field has vanishing optical scalars, but the null geodesic congruence generated by has vanishing twist and shear scalars but nonvanishing expansion scalar
This shows that when looking through departing wavefronts at distant objects, our inertial nonspinning observers will see their apparent size change exactly the same way as the expansion of the timelike geodesic congruence itself.
Read more about this topic: Monochromatic Electromagnetic Plane Wave
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