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Chapter 9 Optical Properties

Chapter 9 Optical Properties. Objectives. Understand principles of: Refraction of light Refractive indexes Polarization of light Birefringence Pleochroism Optical indicatrix Dispersion. The importance of optical properties of minerals. Optical mineralogy:

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Chapter 9 Optical Properties

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  1. Chapter 9Optical Properties

  2. Objectives • Understand principles of: • Refraction of light • Refractive indexes • Polarization of light • Birefringence • Pleochroism • Optical indicatrix • Dispersion

  3. The importance of optical properties of minerals • Optical mineralogy: • study of interaction of polarized light in minerals • NB for: • ID minerals • Reveal characteristics of minerals • Petrographic mineralogy: • Systematic description of minerals as they occur in rocks in THIN SECTION • Basis of petrographic mineralogy: • Isotropy and anisotropy • Isotropy: Homogeneity in all directions • Anisotropy: presence of a preferred orientation • Preferred orientation: concentration of linear or planar, structural or fabric elements with a preferred attitude • Directional interaction of light with crystals

  4. Isotropic/Anisotropic • Isotropic Isometric • Anisotropic Uniaxial Tetragonal Hexagonal Trigonal • Anisotropic Biaxial Orthorombic Monoclinic Triclinic

  5. Waves and light • Visible light is electromagnetic radiation (Fig 9.1) • Electric field E creates magnetic field H (right angles to E) • Propagate at velocity c in vacuum • Light: particle (photon) and wave properties; move in straight direction @ velocity c • Use either property depending on optical effect required • All visible wavelengths together (400-800nm) – white light • Monochromatic light – one wavelength (sodium vapor lamp)

  6. Spectrum of electromagnetic radiation

  7. Refraction • Change in direction of a wave due to a change in velocity • Occur when wave passes between different mediums • Most common example: refraction of light • Also in sound waves, water waves

  8. Refraction of light

  9. Refraction • Described by Snell’s law: • The angle of incidence and the angle of refraction are related to the velocity of the incidence and refracted rays and inversely related to the refractive index of the two mediums of travel • In other words: • When light goes from less dense to denser medium, it changes direction – refracted • Except for 90º incidence angle

  10. Snell’s law READ: Box 9.1 in textbook for more detailed explanation

  11. Refractive Index • A measure for how much the speed of waves is reduced inside a medium compared to inside a vacuum • n = c/v  the velocity ratio • c = velocity of light in vacuum (vvacuum) • v = velocity of light in medium (vmedium) • In mineralogy the medium is a mineral (vmineral) • Thus: nmin = vvacuum / vmineral

  12. Refractive index • Light in mineral: scattered by electrons – time delay observed • Thus: fundamental velocity of the light wave does not change – in reality the denser electron-packing causes a longer path length for the wave • Refractive index increase when number of electrons per unit volume increase  in general when density increase • RI can vary with: • direction of light • wavelength • temperature

  13. Determining the refractive index of a mineral • Used in optical microscopic mineral ID • Minerals has characteristic refractive indices • Make use of liquids with known refractive indices to determine the index of the mineral in question • Determine by means of relief and Becke Line

  14. Determining the refractive index of a mineral

  15. Determining the refractive index of a mineral

  16. Determining the refractive index of a mineral

  17. Polarization • Polarization - describes orientation of oscillations of waves • Transverse waves (light) • Oscillations in plane perpendicular to direction of propagation

  18. Polarization • When a wave of light is filtered to have only one vibrational direction • Vibrations in plane perpendicular to propagation • Under microscope: • Observed as: • Pleochroism – plane polarized light • Birefringence – crossed polarized light

  19. Obtaining polarization • By using polaroid filter • Organic synthetic crystals • The most common method of polarization • Shows strong preferential absorption due to different bonding forces in different crystal directions • Made of material capable of blocking one of two planes of vibration of electromagnetic wave • Thus: filters out one-half of vibrations • Unpolarized light into Polaroid filter  emerges: half intensity; single plane vibrations  polarized light • Other crystals; many minerals also show directional absorption • not complete absorption - only certain wavelengths • Colour changes during rotation in polarized light  PLEOCHROISM – very useful ID tool • Also obtain polarization by: • Reflection • Refraction • Scattering

  20. Birefringence • When a ray of light is split into two separate polarized rays – each with a single vibration direction perpendicular to that of the other ray • Under the microscope: • Observed under crossed polarized light as: • Interference colors • Only in anisotropic minerals

  21. Birefringence/double refraction • Decomposition of a ray of light into two rays (the ordinary ray, ω, and the extraordinary ray, ε) when passing through certain types of material, such as calcite crystals or boron nitride • Only in anisotropic minerals • Uniaxial birefringence: material with two different refractive indices – nω and nε • Biaxial birefringence (trirefringence): material with refractive index of three “equal” values - nα, nβ and nγ • Calculated as the path difference between polarized rays after leaving a crystal of thickness d – also called optical retardation

  22. Birefringence • Incident light refracted into two different paths - split into two beams • Show two images if object is viewed through double refractive crystal • Result of BIREFRINGENCE of light – both beam polarized, but perpendicular to each other • Due to different refractive indices of mineral in the two or three different directions • Two rays can be blocked out individually by filters  Evidence for the wavelike behaviour of light

  23. Birefringence

  24. Birefringence/double refraction • Doubly refracted waves are polarized but separate, vibrating in different planes – no interaction • Need interference to study interference colours and other properties • To get interference – a second polarizer inserted – the analyzer: • Crossed polarizer • Used to analyze the interference effects of light in minerals

  25. NB Terminology • Pleochroism • Uniaxial • Biaxial • Interference colors • Monochromatic light • Relief • Becke line • Plane polarized light • Crossed polarized light • Path difference • Optical retardation • Analyzer • Optical plane • Optical axis • Optical mineralogy • Anisotropy • Polarized light • Crystal • Mineral • Petrographic mineralogy • Preferred orientation • Refraction of light • Refractive index • Birefringence • Ordinary ray • Extraordinary ray • Thin section

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