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WHAT IS OPTICAL PROPERTIES OF MINERALS? WHAT ARE COMPOUND, ELECTRONS, PETROGRAPHIC MICROSCOPE?

  COMPOUND MICROSCOPE: It is a microscope, which multiple lenses use to enlarge the image of a sample. the total amount of magnification calculates by multiplying the magnification of the ocular lens with the magnification of the object lens.


ELECTRONS MICROSCOPE: It is a type of microscope, where electrons are used for observing a sample and making an enlarged image. this kind of microscope produces a much higher magnification.

#Formula of total magnification = (power of the objective) X (power of the eyepiece)

PETROGRAPHIC MICROSCOPE: it is a compound instrument, within which plan polarized light is used for the examination of rock textures and optical properties of minerals.

PARTS OF POLARIZING MICROSCOPE:




1) Mirror

2) Polarizer

3) Condenser

4) Diaphragm

5) Stage with section holders and circular scale

6) Objective

7) Accessory slot for a special plate

8) Analyzer

9) Bertrand lens

10) Eyepiece

HOW DOES POLARIZED LIGHT MICROSCOPY WORK:

Answer: Polarized light microscopy works by converting ordinary light to polarized light. The device which converts the normal light or unpolarized light to the plane-polarized light, such device is termed a polarizer.

What is the function of a polarizer?

Answer: A polarizer consists of long-chain molecules aligned in a particular direction, which work as a filter that filters unpolarized light to plane-polarized light. It passes only the light waves of a particular polarization and blocks the light waves of other polarization.



WHAT IS THE DIFFERENCE BETWEEN POLARIZER AND ANALYZER?

Answer: A polarizer and an analyzer are the parts of an optical instrument. 

the main difference between a polarizer and an analyzer is that polarizer produces plane-polarized light whereas an analyzer is used to detect the light is polarized or not.

WHAT IS THE FUNCTION OF THE BERTRAND LENS IN A MICROSCOPE?

Answer: Bertrand lens is used to make interference figures and assist in aligning a microscope to come up with interference figures.

WHAT IS A THIN SECTION?

Answer: a thin section could be a 30micro meter (0.03 millimeter) thick slice of rock attached to a glass slide with epoxy. Typical thin section slides are 26mm X 46 mm.

OPTICAL PROPERTIES OBSERVED UNDER POLARIZED LIGHT:

1) Color

2) Refractive Index

3) Cleavage

4) Fracture

COLOR: Colors in the minerals are caused because of absorption and emission of light. Among the common rock-forming minerals, Ferro-magnesium minerals and Titanium minerals are strongly colored. Minerals like quartz, K-feldspar, plagioclase feldspar, Feldspathoid, muscovite, sericite don't seem to be in the least colored.

CLEAVAGE: cleavage may be defined as the tendency of minerals to break more easily with smooth surfaces along the planes of weak bonding. Micas, Amphiboles, pyroxene, calcite, and fluorite exhibit very characteristic cleavage. Minerals like quartz, nepheline, and cordierite don't show any cleavage.

Fracture: Fracture may be defined as the tendency of minerals to break with an irregular surface because of the absence of cleavage. Garnet and olivine show irregular cracks.

PlEOCHROISM: Pleochroism is an optical phenomenon in a thin section, in which a mineral shows variation in colors due to differential absorption of the wavelength of light in different directions. 

1) ISOTROPIC SUBSTANCE: In an Isotropic substance, the absorption of light is the same in every direction and there is no vibration of intensity and color of the light during the rotation of the stage. Accordingly, the isotropic substance is non-pleochroic.

     

2)   ANISOTROPIC SUBSTANCE: these minerals show distinct Pleochroism, but it depends on the crystallographic orientation of the section cut from the mineral. A different section of the same mineral shows a different degree of Pleochroism.   

   On the basis of the number of directions along which no double refraction occurs, anisotropic minerals may be divided into two groups:

a) Uniaxial minerals: Minerals of tetragonal and hexagonal system, which possess only one optic axis.  

b) Biaxial minerals: Minerals of the orthorhombic, monoclinic, and triclinic systems possess two optic axes, along which no double refraction occurs.

Optic axis: It is a line in a doubly refracting medium, which is parallel to the direction in which all components of plane-polarized light move with the same speed. 

Some Pleochroism minerals:

Orthopyroxenes (Hypersthene – shade of moderate pink to green)

Titan Augite (shades of pinkish-brown) 

Biotite (shade of tan brown)

Phlogopite (deep orangish-red)

Tourmaline (shades of honey brown to green)

Refractive Index: Refractive index of a medium may be defined as the ratio of the speed of light in Vacuum or air to the ratio of the speed of light in the given medium. 

   To measure the optical density we used the refractive index. 


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