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