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Reflected light Optics and Preparation of polished ore samples.pptx
- 1. Reflected light Opticsand Preparation of polished ore samples
- 2. Violet (380 nm)----------------------Red (780 nm) Opaque minerals are studied in reflected light & that is termed as ore microscopy. Commonly limited to visible spectrum of light Light produced by a single wave wavelength is called monochromatic and light containing variety of wavelengths are polychromatic. white light from sun or artificial source comprises of all the visible spectrum. Frequency wavelength speed of light
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- 5. Refractive Index It isthe relation between the velocity of light in vaccum to that in material. Refractive Index (RI), n = Vvaccum Vmaterial C= Vvaccum = 3.00x108 m/s which is constant So, the RI of the material is characteristic of the light velocity in that material. n ∝ 1 Vmaterial
- 6. Snell’s Law ni nR As theRI of light in air or vaccum is 1 nR sin i sin R = n ∝ 1 Vmaterial As In this case the light ray would bend towards the normal. In other words when the ray bends towards the normal the velocity decreases and the medium has higher RI. nR sin i sin R ni = = Vi VR
- 7. Normal Light vsPolarised Light When light vibrates in only one plane of vibration, it is called Polarized light. Normal light vibrates in all possible planes that contain the light ray When light vibrates in all possible planes of vibration, it is called Unpolarised light. light source light ray waves travel from source to eye wavelength, l amplitude, A plane of vibration vibration direction propagation direction
- 8. Polarisation canbe achieved by: Reflection Refraction Absorbation Transmission using polaroids The process of transforming unpolarized light into polarized light is known as polarization When angle between reflected and refracted rays is 90° the reflected and refracted rays are completely plane polarised. Remember the Snell’s Law.
- 10. Isotropic and Anisotropicminerals Isotropic minerals show optical behaviour that is independent of the direction of light propagation i.e., they are identical in all directions . c- c+ b+ b- a- a+ Isotropic minerals a=b=c & α=β=γ=90° α β γ
- 11. Anisotropic minerals havemore than one characteristic RI which are directionally dependent. Anisotropic minerals Uniaxial minerals Biaxial minerals This classification is based on optic properties Hexagonal (Quartz, calcite, apatite, tourmaline) Orthorhombic (Opx, Orthoamphiboles (Anthophyllite- Gedrite), Olivine, Sillimanite, Andalusite, Barytes) Tetragonal (Zircon, rutile) Monoclinic (Kfs, micas, Cpx, clinoamphiboles) Triclinic (Plagioclase, Kyanite)
- 12. c ε ω Double Refraction Mutually perpendicularvibrations of variable RI and thus velocity The ray with refractive Index, ε is the ray that travels parallel to c-axis and is called as extraordinary ray. The ray with refractive index, ω is the ray that travels perpendicular to the c-axis and is called as ordinary ray. InTransmitted Light Microscopy
- 14. In reflected lightmicroscopy There are three different but related types of polarization: 1. plane polarization 2. circular polarization and 3. elliptical polarization. Plane Polarized
- 15. Circular polarized Light Two waves with equal amplitude vibrate at right angles to each other with one wave retarded ¼ λ relative to the other. Vector sum of these two waves is a helix surface that resembles the threads on a screw, whose cross section is circular (When viewed along the direction of propagation). Circular Polarized
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- 17. Elliptical polarized Light produced in the same manner as circular polarized light except the two waves that produce it are retarded relative to each other by a value different than ¼ λ. The result is still a helix, but instead of being circular in cross section, the helix is elliptical. Elliptical Polarized
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- 19. Light path inthe reflected light microscope
- 20. Preparation of samplesfor Ore Microscopy Prime criteria for preparation of sample for ore microscopy: Should be: free of scratches Thermal and mechanical modifications of sample surface Relief (uneven surface due to hard phases being worn away less than soft) Casting of samples Samples are cut with wet diamond saw to avoid heating of sample They are then grinded to provide a flat surface
- 21. Samples are thencasted generally in circular molding (2.5 to 5 cm diameter) material These molds (made of plastic, polyethene or metal) are given a thin coating of non-reactive lubricating agent (e.g., silicon stopcock grease or vaseline). Samples are then set in cold- setting epoxy resins or plastics mounting media were no heat or pressure is required. Casting of samples
- 22. Grinding and polishingof samples Once the molded samples are ready after drying they are grinded on rotary equipment. Grinding removes surface irregularities, casting resin over the surface of the sample, reduce thickness, prepare smooth surface. Grinding is done using emery or carborundum powder. Grinding and Polishing Machine Abrasives with grain size 120, 240, 400, 600 mesh are used successively for few minutes. Followed by longer duration of grinding using 1000-1200 mesh carborundum on glass
- 23. Polishing isdone on a hard napless cloth without excessive weight pressuring down with surface sufficiently just lubricated with 15 μm 6 μm abrasives of aluminum oxide/chromic oxide/magnesia/cerium oxide can be used for polishing on napped cloth (e.g., velvet). Thin-cum-Polished sections Polishing of uncovered surface of a thicker thin section (30-40 microns) Doubly Polished thin section Samples are polished and then glued and then again polished on other side.