Birefringence and Birefringence and InterferenceInterference

Lecture 2Lecture 2

BirefringenceBirefringence BirefringenceBirefringence, or , or

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

BirefringenceBirefringence The two rays travel at The two rays travel at

different speeds, and vibrate different speeds, and vibrate in perpendicular directionsin perpendicular directions

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

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

The extraordinary ray The extraordinary ray vibrates in a direction that vibrates in a direction that would connect it with the would connect it with the ordinary rayordinary ray

Sign of BirefringenceSign of Birefringence If the ordinary ray is faster than the If the ordinary ray is faster than the

extraordinary ray, then the mineral is extraordinary ray, then the mineral is positivepositive

If the ordinary ray is slower than the If the ordinary ray is slower than the extraordinary ray, then the mineral is extraordinary ray, then the mineral is negativenegative

Birefringence and the PLMBirefringence and the PLM

Birefringence and the PLMBirefringence and the PLM

Retardation = Birefringence x Thickness

Interference and ColorInterference and Color Each color has a different wavelengthEach color has a different wavelength

If all light is retarded by the same distance, then If all light is retarded by the same distance, then each color of light will be affected differentlyeach color of light will be affected differently

Some colors may increase in intensity and some Some colors may increase in intensity and some may decreasemay decrease

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

Interference Color ChartInterference Color Chart

The Optic AxesThe Optic Axes An optic axis is a straight line through a mineral An optic axis is a straight line through a mineral

along which light does not diverge into two along which light does not diverge into two separate raysseparate rays

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

Along the optic axis the mineral behaves as if it Along the optic axis the mineral behaves as if it were isotropic (no retardation)were isotropic (no retardation)

Uniaxial MineralsUniaxial Minerals Some anisotropic Some anisotropic

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

Biaxial MineralsBiaxial Minerals Most anisotropic Most anisotropic

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

Orthoscopic vs Conoscopic LightOrthoscopic vs Conoscopic Light

Conoscopic LightConoscopic Light

Uniaxial Interference FigureUniaxial Interference Figure

Key TermsKey Terms BirefringenceBirefringence Ordinary rayOrdinary ray Extraordinary rayExtraordinary ray RetardationRetardation InterferenceInterference Optic axisOptic axis UniaxialUniaxial BiaxialBiaxial

OrthoscopicOrthoscopic ConoscopicConoscopic Interference Interference

FigureFigure IsochromeIsochrome IsogyreIsogyre MelatopeMelatope