Minerals and Their Physical Properties

Birefringence and Interference Lecture 2 Birefringence Birefringence,

or double refraction, is the decomposition of a ray of light into two rays when it passes through anisotropic materials

Birefringence The two rays travel at different speeds, and vibrate in perpendicular directions

One ray travels straight through the crystal and is called the ordinary ray

The other ray is refracted through the crystal and is called the extraordinary ray

The extraordinary ray vibrates in a direction that would connect it with the ordinary ray Sign of Birefringence

If the ordinary ray is faster than the extraordinary ray, then the mineral is positive If

the ordinary ray is slower than the extraordinary ray, then the mineral is negative Birefringence and the PLM

Birefringence and the PLM Retardation = Birefringence x Thickness Interference and Color Each

color has a different wavelength If all light is retarded by the same distance, then each color of light will be affected differently

Some colors may increase in intensity and some may decrease The

result is that the light that results when the rays recombine will have a distinct color due to the interference from a given retardation Interference Color Chart The Optic Axes

An optic axis is a straight line through a mineral along which light does not diverge into two separate rays Corresponds

to an axis of symmetry such that the speed light would be the same no matter what direction the ray vibrates Along

the optic axis the mineral behaves as if it were isotropic (no retardation) Uniaxial Minerals Some anisotropic

minerals (such as calcite and quartz) have only one optic axis, and so are called uniaxial Biaxial Minerals

Most anisotropic minerals (such as muscovite) have only two optic axes, and so are called biaxial

Orthoscopic vs Conoscopic Light Conoscopic Light Uniaxial Interference Figure

Key Terms Birefringence Orthoscopic Ordinary

Conoscopic ray Extraordinary ray Retardation Interference Optic axis

Uniaxial Biaxial Interference Figure Isochrome

Isogyre Melatope