Polarizing microscopes are sometimes referred to as petrographic microscopes due to their use by petrologists, mineralogists, geophysicists, soil scientists, crystal chemists, and materials engineers to study the origin, composition, distribution and structure of rocks. A polarizing microscope is a dedicated high magnification microscope equipped with polarizing light filters, known as the polarizer and analyzer, a rotating specimen stage that makes it easy to view polarized light in the specimen, and a Bertrand lens. It is common to take a basic stereo microscope or biological microscope and add simple polarizing light filters to these microscopes. By adding polarizing filters to these microscopes (the analyzer is placed in the microscope head and an analyzer is placed over the microscope light source and when rotated, polarized light is created) a very basic version of a polarizing microscope is created. Polarizing microscopes use birefringent (double refraction) optical properties to study anisotropic materials (solid substances with several refractive indices).