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For some years I've had a nagging question about the details of how a circular polarizer, as used by photographers, actually works. After a google search, I see lots of sites saying that the filter consists of a linear polarizer and a quarter wave plate. The wave plate is supposed to "circularize" the incident linearly polarized light. This is desirable because the optics in an SLR camera (in particular, the partially transmitting mirror) are designed to have equal amounts of the two linear polarization components. Both randomly and circularly polarized light will work; linearly polarized light will not.
Okay, here is my problem: wave plates are designed to work at specific, discrete wavelengths. But visible light is comprised of a continuous spectrum, covering nearly a factor of two range of wavelengths (400 to 700 nm). So if a specific wavelength in that range gets circularly polarized, other wavelengths will not be; the "quarter wave" plate will be the wrong thickness, and give the wrong amount of relative phase shift, for most wavelengths in the visible range.
I have thought of two possible ways in which these filters could work:
Does anybody know which of these cases correctly describes circular polarizers? Or is something else going on?
Okay, here is my problem: wave plates are designed to work at specific, discrete wavelengths. But visible light is comprised of a continuous spectrum, covering nearly a factor of two range of wavelengths (400 to 700 nm). So if a specific wavelength in that range gets circularly polarized, other wavelengths will not be; the "quarter wave" plate will be the wrong thickness, and give the wrong amount of relative phase shift, for most wavelengths in the visible range.
I have thought of two possible ways in which these filters could work:
1. the wave plate produces a λ/4 relative shift at about 550 nm, in the center of the visible spectrum. Light of other wavelengths are elliptically polarized, but it still works well enough for the purpose at hand.
2. the wave plate produces a relative shift of nλ, where n is a large number (not necessarily an integer) for visible wavelengths. For some wavelengths, n will be an integer ±¼ and the light is truly circularly polarized. But in general different wavelengths are polarized elliptically. The mix of the two linear polarizations is pretty equal, on average, so this does not present a problem.
2. the wave plate produces a relative shift of nλ, where n is a large number (not necessarily an integer) for visible wavelengths. For some wavelengths, n will be an integer ±¼ and the light is truly circularly polarized. But in general different wavelengths are polarized elliptically. The mix of the two linear polarizations is pretty equal, on average, so this does not present a problem.
Does anybody know which of these cases correctly describes circular polarizers? Or is something else going on?