Polarizing microscope

A microscope that uses controlled incident and analysed polarization of an electromagnetic wave to study specimens. Crossed polarizers reveal orientation-dependent phase retardation in a birefringent specimen.

Rotating a uniform crystallite around a fixed propagation direction changes the crossed-polar intensity factor ${\sin }^2(2\alpha )$. Extinctions repeat every90degrees and maxima occur45degrees away. Similar interference colour can coexist with different brightness if retardation is similar but azimuth differs.

A conoscopic interference pattern is the direction-dependent pattern obtained when a polarizing microscope examines convergent illumination passing through a birefringent specimen. Different positions in the observed angular pattern correspond to different propagation directions through the specimen, and hence to different accumulated phase retardations.

A thin crystal between crossed polarizers can therefore show bright and dark bands even with nearly uniform thickness. Inspecting the angular pattern, rather than an ordinary image of specimen texture, helps identify optical-axis directions. The pattern depends on crystal orientation, thickness and wavelength; it is not a map of surface height.

An extinction pattern with four dark arms produced, for example, by a radial arrangement of local optical principal directions between crossed polarizers. The local intensity factor ${\sin }^2(2\phi )$ vanishes along the polarizer axes. Such a radial-material pattern need not be a conoscopic interference pattern.