Polarized Light Microscopy
Entry by Robin Kirkpatrick, AP 225, 2011
Polarized light microscopy is a contrast enhancement technique that takes advantage of the the sample's birefringence. Unpolarized light passes through a polarizing filter, with it's vibration azimuth oriented perpendicular to that of the analyzing filter. As a result, in absense of a bifrefringent sample, no light passes through the system. Plane polarized light passes through the birefringent sample, which results in ordinary and extraordinary wavefronts. These wavefronts are mutually perpendicular, and travel with different wave velocities and are intially out of phase. Passage through the analyzer (the second polarizing filter) results in constructive and destructive interference. This technique is illustrated in the figure below.
Polarized light microscopes in general are equipped with other features, such as
• Strain Free Objectives and Condensers - Stress in the glass can result in a birefringent lens, resulting in undesirable effects.
• Compensator and Retardation Plates - These are used to enhance optical path differences.
Polarized light microscopy has a variety of applictions in geology, chemistry and biology. One of the most well known manifestations of polarized light microscopy is DIC (differential interference contrast microscopy), but can be used to determine the optical axis of samples, and to determine whether or not a material is anisotropic.