= Solution
For <two-element polarization identification>, assume ideal elements and use an <intensity> detector. For each source, test both element orders while rotating their relative axes. \b[The circular source followed by the <quarter-wave plate> and then the <polarizer> is the combination that gives full <intensity> modulation.] The plate converts circular <light> to linear <light>, and the downstream <polarizer> then obeys <Malus's law>.
If the <polarizer> comes first, it transmits half the circular source's <intensity> and the downstream lossless plate cannot change that <intensity>. <Unpolarized light> also gives constant half-intensity in either order: a wave plate leaves it unpolarized, while a <polarizer> followed by a wave plate has no final analyzer. Thus the modulating source is circular, its first element is the <quarter-wave plate>, and its second is the <polarizer>; the remaining source is unpolarized. Testing both orders avoids needing to know either source's original brightness.
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