Petrographic Microscopy Questions PDF

51-What is the primary purpose of a petrographic microscope in geology? a) To observe large-scale structures in rocks b) To analyze the chemical composition of rocks c) To examine small-scale structures and determine mineralogy d) To measure the density of rocks 52-What is the first step in prepari...

51-What is the primary purpose of a petrographic microscope in geology? a) To observe large-scale structures in rocks b) To analyze the chemical composition of rocks c) To examine small-scale structures and determine mineralogy d) To measure the density of rocks 52-What is the first step in preparing a rock sample for examination under a petrographic microscope? a) Grinding the rock b) Making a thin section c) Mounting the rock on a glass slide d) Cutting the rock into slab 53-What is the standard thickness for a thin section used in petrographic microscopy? a) 0.1 millimeters b) 0.5 millimeters c) 0.03 millimeters d) 300 microns 55-What happens when a rock sample is placed between the two polarizing filters in a petrographic microscope? a) The sample absorbs all light. b) The sample bends the light, creating interference colors. c) The sample reflects light in different directions. d) The sample remains transparent 56. Which property of minerals refers to the difference in how they stand out from each other under a microscope? a) Pleochroism b) Relief c) Twinning d) Extinction 58. In a petrographic microscope, what does it indicate when quartz grains in a sample go completely dark? a) They are highly reflective. b) They are under high pressure. c) They are twinned. d) They are experiencing extinction Which of the following formulas correctly represents total magnification in a microscope? A. Total magnification = Objective lens magnification ÷ Ocular lens magnification B. Total magnification = Objective lens magnification × Ocular lens magnification C. Total magnification = Objective lens magnification + Ocular lens magnification D. Total magnification = Objective lens magnification - Ocular lens magnification 1 Which of the following describes the correct setup of a petrographic microscope? A. Analyzer parallel to the polarizer B. Analyzer at 45° to the polarizer C. Analyzer perpendicular (90°) to the polarizer D. Analyzer at 90° to the light source 68. What type of minerals are extinct at all orientations under cross-polarized light? A) Anisotropic B) Opaque C) Pleochroic D) Isotropic 67. How many extinction positions occur in 360 degrees rotation of cross-polarized light? A) Two B) Three C) Four D) Five 66. What is observed under cross-polarized light? A) Interference colors B) Pleochroism C) Relief D) Becki line movement 69. What property is used to calculate birefringence? A) Pleochroism B) Index of refraction C) Becki line movement D) Retardation 70. How are interference colors related to retardation? A) Directly proportional B) Inversely proportional C) Linear relationship D) Non-linear relationship 80 What does the accessory plate help determine in minerals? A) Index of refraction B) Pleochroism C) Slow and fast rays D) Becki line movement 62. What property causes some minerals to change color when the stage is rotated? A) Pleochroism B) Isotropy C) Anisotropy D) Birefringence 2 63. What is the result of a difference in the index of refraction between a mineral and its surroundings? A) Birefringence B) Relief C) Pleochroism D) Interference colors What factor primarily determines the relief of a mineral grain in a mounting medium? A. The color of the mineral B. The refractive index difference between the mineral and the medium C. The size of the mineral grain D. The crystal structure of the mineral 2. True/False Question: High relief occurs when there is a small difference (≈ 0.04) in the refractive index between the mineral and the mounting medium. Correct Answer: False What does the term "relief" refer to in mineralogy? A.The hardness of a mineral in a mounting medium B. The visibility degree of mineral grains in a mounting medium C. The color of a mineral in polarized light D. The orientation of a mineral under a microscope Which direction does the Becke line move when the stage is lowered for a mineral with positive relief? A) Into the material B) Out of the material C) Remains stationary D) Moves sideways How is relief described in terms of positivity or negativity? A) Positive or negative B) High or low C) Faint or distinct D) Thick or thin 66. What is observed under cross-polarized light? A) Interference colors B) Pleochroism C) Relief D) Becki line movement 3 67. How many extinction positions occur in 360 degrees rotation of cross-polarized light? A) Two B) Three C) Four D) Five 68. What type of minerals are extinct at all orientations under cross-polarized light? A) Anisotropic B) Opaque C) Pleochroic D) Isotropic What is a key characteristic of isotropic mineral? A. They show different velocities of light in all directions. B. They exhibit double refraction. C. They have the same velocity of light in all directions. D. They appear bright under crossed polarization. -Isotropic minerals are dark under crossed polarization. true -Isotropic materials show double refraction and have the same velocity of light in all directions. false Which of the following is a characteristic of anisotropic materials? A. Light velocity is the same in all directions. B. They show no double refraction. C. Light velocity differs in different directions. D. They are dark under crossed polarization Anisotropic minerals exhibit double refraction, splitting light into two groups of rays with different velocities. Answer: True -What is the reason for the different light velocities in anisotropic materials? A. Uniform chemical bonding. B. Symmetry equal to isometric systems. C. Chemical bonding and electron cloud vibration differ in different directions. D. No interaction with light. 4 Anisotropic materials include minerals from the cubic crystal system. Answer: False -Double refraction in anisotropic materials refers to the splitting of light into two rays with different velocities Answer: true -Anisotropic materials have varying light velocities in different directions, exhibit double refraction, and are colored under crossed polarization, unlike isotropic materials. Answer: true -Anisotropic minerals belong to crystal systems such as tetragonal, hexagonal, trigonal, orthorhombic, monoclinic and triclinic Answer true -Anisotropic minerals have lower symmetry than the isotropic Answer true -In anisotropic minerals, the vibration of electron clouds is not the same in all directions Answer true -In anisotropic minerals, the chemical bonding differs in different directions Answer true Under crossed Nicols, when no sample is placed on the stage, the analyzer absorbs the light coming from the polarizer, resulting in a dark view. Answer true What happens under crossed nicols if no sample is placed on the stage? A. The view becomes bright. B. The view remains dark. C. Light passes through unchanged. D. Polarization is unaffected. 5 -When no sample is on the stage, the light coming from the polarizer is absorbed by the analyzer because their vibration directions are perpendicular to each other, preventing light transmission. Answer true -The dark view under crossed nicols occurs because the vibration directions of the polarizer and analyzer are perpendicular. Answer true Straining of isotropic materials may change them to anisotropic. Answer true -What effect can straining have on isotropic materials? A. It makes them harder but retains isotropy. B. It changes their optical properties to anisotropic. C. It does not affect their optical properties. D. It turns them into isotropic crystals. -Straining of isotropic materials may alter their optical properties, transforming them into anisotropic materials. Answer true As the stage lowered, Becke line propagates towards the higher n material Answer true What causes interference colors to appear between crossed nicols? A.Reflection of light from the sample surface. B. Absorption of light by the analyzer. C. Splitting of light into two rays with different velocities. D. Scattering of light within the polarizer. Interference colors are produced between crossed nicols due to light being split into two rays. Answer : true What factors influence retardation (Δ) in anisotropic minerals? A. Sample thickness and isotropy. B. Birefringence and refractive index. C. Sample thickness and birefringence (ns−nf). D. Symmetry and isotropy. 6 Retardation (Δ) is the distance between the fast and slow rays and depends on sample thickness and birefringence. Answer: True -Birefringence is the difference in refractive indices between the slow and fast rays. Answer: True -Retardation (Δ) is the distance between the slow and fast rays. Answer: True -Only anisotropic minerals show interference colors Answer: True -Light path along optic axis = Zero birefringence Answer: True -The difference between the indices of the slow and fast rays (birefringence) depends on the light path through the mineral and the light wavelength. Answer: True -The variety of interference colors of the same mineral is mainly due to different orientations of the grains relative to the cut section, which affects the interaction of light with the mineral. Answer: True -Uniform grain orientation relative to the cut section ensures that all grains similarly interact with light, leading to consistent interference colors under crossed Nicols Answer: True -Different grain orientation of same mineral relative to the cut section lead to variety of interference colors of the same mineral. Answer: True -Which of the following is true regarding the interference colors of the same mineral? A) Uniform grain orientation relative to the cut section leads to a variety of interference colors. B) Different grain orientations relative to the cut section result in a variety of interference colors. C) Grain orientation does not affect interference colors. D) Interference colors are solely determined by the mineral's composition, not its orientation. 7 -Quartz interference color is 1st order white (grey) or 1st order yellow. Answer: True -How can the birefringence of a mineral be determined using the Interference Color Chart in a standard thin section? A) Measure the thickness of the mineral and divide it by its refractive index. B) From the 0.03 mm horizontal line, and for a known interference color, read up along the appropriate diagonal line to get (birefringence) C) Identify the mineral's optical sign and compare it to the chart's color scheme. D) Match the mineral's interference color directly to its density on the chart. E) look down along a diagonal line to its intersection with the 0.03 mm line to see what the interference color should be in a standard thin section - isotropic mineral extinct one every 90˚ of the stage rotation. False -What happens when the resolved vector of the vibration directions of the slow and fast rays is perpendicular to the vibration direction of the analyzer in polarized light microscopy? A) The mineral appears bright and colorful. B) The mineral shows interference colors. C) All light is absorbed, and the mineral appears dark (extinction). D) The mineral exhibits double refraction. -when the resolved vector of the vibration directions of the slow and fast rays is parallel to the vibration direction of the analyzer, all light will be transmitted from analyzer -Bright looking. True -What is the extinction angle (EA) in polarized light microscopy? a) The angle between the cleavage or mineral elongation and the extinction direction. b) The angle between the mineral’s cleavage and its crystallographic axis. c) The angle between the mineral’s axis of rotation and the vibration direction of the analyzer. d) The angle between two intersecting cleavage planes. -The visibility degree of a mineral grain in the mounting medium is the definition of: a) Interference color. b) Optic sign. c) Relief d) Pleochroism 8 - ……….. materials show the same velocity of light in all directions. a) Isotropic b) Anisotropic c) Opaque -……………affect light polarization and appear bright or colored under crossed Nicols, except at certain directions. a) Isotropic b) Anisotropic c) Opaque -2) …..…is a band of light around the mineral grain boundary. a) Becke line b) Retardation c) Birefringence 1) Some minerals are commonly associated with certain rocks or mineral deposits and others are rarely found together. T 2) The greater the difference in refractive index between the oil and the mineral, the brighter the Becke line and the lower the relief of the mineral. F 3- The visibility degree of mineral grains in a mounting medium depends on the difference in the refractive indices (n). T 4) when a mineral is immersed in a medium of the same refractive indices (n), the mineral is distinguished. F 5) As the stage lowered, the Becke line propagates towards the lower n material. F 6) Garnet and fluorite are isotropic minerals. T 7) Isotropic minerals show double refraction. F 8) No double refraction occurs when the light path along the optic axis of a mineral. T 11) The rock mainly consisting of pyroxene and plagioclase is: a) Peridotite b) Gabbro c) Limestone d) Dunite Pleochroism is the change in the color of a mineral as the stage is rotated under cross- polarized light. False 12) The rock mainly consisting of Olivine is: a) Peridotite b) Gabbro c) Limestone d) Dunite What type of extinction occurs when the extinction angle (EA) is 0°? a) Parallel extinction b) Inclined extinction c) Symmetrical extinction 9 d) Undulatory extinction Answer: a) Parallel extinction Which extinction type has an extinction angle (EA) between 0° and 90°? a) Parallel extinction b) Inclined extinction c) Symmetrical extinction d) No extinction Answer: b) Inclined extinction What type of extinction occurs when the extinction angle (EA) is the same on both sides, at 45°? a) Undulatory extinction b) Symmetrical extinction c) Inclined extinction d) Parallel extinction Answer: b) Symmetrical extinction What is the name of the extinction type that appears wavy due to deformation? a) Parallel extinction b) Inclined extinction c) Symmetrical extinction d) Undulatory extinction Answer: d) Undulatory extinction What type of extinction occurs when there is no cleavage or elongation to measure? a) Parallel extinction b) Inclined extinction c) Symmetrical extinction d) No extinction Answer: d) No extinction -All minerals show pleochroism. false -Biotite show a strong pleochroism. True -A grain of mineral produces 250 nm of retardation; after inserting the mica plate, the interference colors increase in the NW-SE quadrants. In this case, the total retardation is equal to: a) 800 nm b) 397 nm c) 550 nm What is the orientation of the accessory plate in a microscope? a) NE-SW b) NW-SE c) N-S d) E-W Answer: b) NW-SE 10  Which direction does the fast ray of the accessory plate vibrate? a) Perpendicular to the length of the plate b) Parallel to the length of the plate c) Diagonal to the plate d) Along the NW-SE axis Answer: b) Parallel to the length of the plate Which direction does the slow ray of the accessory plate vibrate? a) Parallel to the width of the plate b) Across the width of the plate c) Along the NW-SE axis d) Perpendicular to the NW-SE axis Answer: b) Across the width of the plate What is the primary use of an accessory plate in microscopy? a) To determine mineral density b) birefringence and optical characteristics of minerals c) To identify mineral cleavage d) To analyze fluorescence properties Answer: b) birefringence and optical characteristics of minerals What can be determined using an accessory plate in a microscope? a) The hardness of minerals b) The type of ray (slow or fast) in a mineral c) The chemical composition of minerals d) The specific gravity of minerals Answer: b) The type of ray (slow or fast) in a mineral What happens to the interference color when the accessory plate is inserted in the microscope and the fast and slow rays vibrations of the mineral are parallel to those in the plate? a) The interference color decreases b) The interference color remains unchanged c) The interference color increases d) The interference color disappears Answer: c) The interference color increases When the fast and slow rays vibrations of a mineral are parallel to the accessory plate's vibration directions, how is the total retardation (Δtot) calculated? a) Δtot = Δm - Δp b) Δtot = Δm + Δp c) Δtot = Δm x Δp d) Δtot = Δm / Δp Answer: b) Δtot = Δm + Δp Under what condition does the total retardation (Δtot) become the sum of the mineral and accessory plate retardations? a) When the accessory plate is removed b) When the fast and slow rays vibrations of the mineral are parallel to those of the accessory plate 11 c) When the mineral and accessory plate vibrate perpendicular to each other d) When the interference color decreases Answer: b) When the fast and slow rays vibrations of the mineral are parallel to those of the accessory plate What does the accessory plate help determine in minerals? A) Index of refraction B) Pleochroism C) Slow and fast rays D) Becki line movement 74. What is used to distinguish between the slow and fast rays of a mineral? A) Index of refraction B) Becki line movement C) Accessory plate D) Diaphragm adjustment 81. How is orientation determined for the fast and slow rays of a mineral? A) By the index of refraction B) By the Becki line movement C) By observing extinction positions D) By using the accessory plate Twinkling is a change of the relief as the stage is rotated in plane-polarized light. True Calcite is characterized by twinkling true The twinkling phenomena is a common characteristic in calcite. true 12