Polarised light microscopy

Polarised light microscopy uses plane-polarised light to analyse structures that are birefringent; structures that have two different refractive indices at right angles to one another (e.g. cellulose microfibrils). Normal, un-polarised, light can be thought of as many sine waves, each oscillating at any one of an infinite number of orientations (planes) around the central axis. Plane-polarised light, produced by a polar, only oscillates in one plane because the polar only transmits light in that plane.

The polarised light microscope must be equipped with both a polarizer, positioned in the light path somewhere before the specimen, and an analyser (a second polarizer), placed in the optical pathway after the objective rear aperture. Image contrast arises from the interaction of plane-polarized light with a birefringent (or doubly-refracting) specimen to produce two individual wave components that are each polarized in mutually perpendicular planes. The velocities of these components are different and vary with the propagation direction through the specimen. After exiting the specimen, the light components become out of phase, but are recombined with constructive and destructive interference when they pass through the analyzer.

Polarised light microscopy can be used to measure the amount of retardation that occurs in each direction and so give information about the molecular structure of the birefringent object (e.g. orientation).