= Waveplate
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= Wave plate
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A <waveplate> uses <birefringence> to change <polarization of an electromagnetic wave> through <phase retardation>. Its fast axis has the smaller <refractive index>, hence the larger <phase velocity>; its slow axis has the larger index. Consider normal incidence with the <optic axis> in the plate face, so both components follow the same straight path. A <linear polarization> at $45^\circ$ to these axes excites equal <wave amplitudes>.
A complementary experiment uses two sheet <polarizers> with a cellophane sheet between them. First cross the <polarizers> and remove the sheet: the field is dark. Insert and rotate the sheet about the beam: light returns at intermediate orientations, but extinction recurs at two perpendicular orientations. The sheet has changed the <polarization of an electromagnetic wave>, without requiring a resolved double image. With white <light>, different <wavelengths> can produce different transmitted colors.<The Feynman Lectures on Physics, Volume I by Richard Feynman and others>[I §33–3, Fig. 33–3 and the paragraphs describing removal, rotation and white-light colors of cellophane]{ref}. Brightness alone does not establish <circular polarization>: an ideal rotating linear <polarizer> also gives orientation-independent <light intensity> for <unpolarized light>.
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