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holger_merlitz
sage
Reged: 02/08/04
Posts: 282
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Phase shift of polarization!
02/15/07 10:43 PM
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Hello Peter,
Your assumptions about the origin of the phase shift are unfortunately incorrect. In fact, what happens is the following:
During any reflection, including the process of total reflection inside a prism, unpolarized light becomes partially polarized. Obviously, this is the case in both Porro and roof prisms. The difference is: Using Porro prisms, the light will finally enter the eye being partially polarized, which does not matter. In a roof prism, however, the beam is split in two partial beams, as you have correctly described above. Now one has to consider that the polarization of light defines a kind of direction; during the following reflections, the direction of this polarization vector is changed, but unfortunately, it is changed differently in both partial beams. Once they are rejoined, both polarization vectors do not coincide, they define an angle between each other (which is called 'phase shift', or geometric phase, or Berry phase). As a result, an interference takes place which is disturbing the airy disk of a point-like source. How it looks like, is shown here:
http://www.zbirding.info/zbirders/blogs/sing/archive/2006/08/09/189.aspx
Therefore, the phase shift has nothing to do with different path lengths of partial beams - it is a shift of directions their polarization vectors. Even perfectly constructed roof prisms would exhibit such a geometric phase. In turn, the phase shift coating cannot correct for inaccuracies of the prism's geometry, but it makes both polarization vectors coincide when the partial beams are rejoined.
Best,
Holger
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