Polarizer Rotation and Specimen Birefringence
When a birefringent material is placed between crossed polarizers in an optical microscope, light incident upon the material is split into two component beams whose amplitude and intensity vary depending upon the orientation angle between the polarizer and permitted vibration directions of the material. This tutorial explores the effects of polarizer rotation on specimen birefringence as observed in a polarized light microscope.
To operate the tutorial, first select a specimen from the pull-down menu labeled Choose A Sample. When the tutorial initializes, the specimen will have both focus and illumination intensity values chosen at random. Use the Focus slider to bring the specimen into focus and the Intensity to optimize specimen illumination. The Polarizer Rotation slider can be used to rotate the position of the polarizer from 0 to 180 degrees. Alternatively, the small blue arrow buttons will change polarizer rotation angle in discrete increments. As the polarizer is rotated, interference colors arising from specimen birefringence will change. The polarizer can be toggled into and out of the light path with the Analyzer radio buttons. Use the Reset button to reinitialize the applet to a different set of random values.
Detection of dichroism or pleochroism can be difficult in materials that are only weakly birefringent or when thin preparations are utilized. In this case, it is often prudent to allow the specimen to maintain a stationary orientation with respect to the microscope optical axis while rotating the polarizer. As the polarizer is rotated, the interference colors displayed by the birefringent material may vary in intensity, but the order (and hue) of the colors does not change. For any given portion of the specimen, there is a position (the extinction position) for which there is zero light intensity visible in the microscope viewport. Maximum transmitted light intensity will occur half way between extinction positions at 45-degree angles. Use the slider to rotate the polarizer to determine exact extinction and maximum light intensity for various portions of the specimens available in this tutorial.
Matthew J. Parry-Hill and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.