Does every day-to-day life reflection cause a phase shift of pi? Or does this occur only in thin-film interferences?


The Lloyd's mirror arrangement to show interference has a dark fringe when two physical path lengths are the same.

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There is a source of light above a flat glass plate.
Light reaches position to the right of the glass plate either having travelled there directly (blue rays) or after being reflected the the glass plate (red rays).
Those reflected rays appear to come from the virtual image of the light source.
The two sources (one real and one virtual) are coherent but because of the $\pi$ phase change of the light after reflection from the glass plate are $\pi$ out of phase.
This means that when the physical path difference is zero a dark fringe is formed.

The Wikipedia article cited above gives a number of application of the Lloyd's mirror arrangement which you may or may not class as "day to day"?

The article does give an interesting example for sound.

The Lloyd mirror effect has been implicated as having an important role in explaining why marine animals such as manatees and whales have been repeatedly hit by boats and ships. Interference due to Lloyd's mirror results in low frequency propeller sounds not being discernible near the surface, where most accidents occur. This is because at the surface, sound reflections are nearly 180 degrees out of phase with the incident waves. Combined with spreading and acoustic shadowing effects, the result is that the marine animal is unable to hear an approaching vessel before it has been run over or entrapped by the hydrodynamic forces of the vessel's passage.


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