In optical mineralogy, the kind of light used in detecting the pleochroism property of the mineral, in studying the interference colors, in locating the direction of vibration, in determining the strength of the double refraction, in measuring extinction angles, in distinguishing whether the mineral section is isotropic or anisotropic, in finding the direction of vibration of the faster and slower rays, in determining relative value of the indices of refraction of the two rays, and in investigating the crystal structure of the section in general is called parallel light.

 

            The kind of light used by mineralogists in distinguishing whether the mineral is biaxial or uniaxial, or whether the section that appears to be isotropic is really isotropic, or only perpendicular to an optic axis, and in determining the optical character, grade summary, dispersion, as well as axial angle is called convergent light.