Many years ago I regularly drove along the M4 between London and Bristol, listening to the radio. There were places along the road where the signal was considerably weaker, and the volume decreased. This may still happen in places, but it did so then because there was a transmitter near Slough. My radio was picking up a signal both directly from the transmitter and from its mirror image ‘in’ the road.
This is a well-known phenomenon with light called Lloyd’s mirror effect, which was first described in the 1830s. It is an interference effect, which is observable because the source and its mirror image emit waves of the same wavelength, and are phase-locked – coherent sources. Where the direct and reflected waves meet will be a position of constructive interference if they are in phase, but a position of destructive interference if π radians (180°) out of phase.
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