Syllabus Optical Mineralogy

CRYSTALLOGRAPHY UNIT IV UNIT-IV: SYLLABUS OPTICAL MINERALOGY : Light: Corpuscular, electromagnetic and quantum theories. Ordinary light and plane polarized light. Refractive index an its determination: Relief method, Becke line, Central illumination, and Oblique illumination methods. Isotropism, isotropic minerals and isotropic ray velocity surface. Behaviour of light in isotropic minerals. Petrological Microscope and its parts-optical accessories and their uses:Quartz wedge, Gypsum plate and Mica plate. Study of Isotropic minerals using the petrological microscope: properties of isotropic minerals under parallel Nicol conditions. LightCorpuscular A source of light continuously emits tiny elastic particles called corpuscles. These particles or the corpuscles moves with high velocity as that of light and gets scattered in all directions of light. This theory says that the velocity of light changes with the change in density of the medium in which it is used. This theory could explain three main phenomena of light that is reflection, refraction, and rectilinear propagation of light. This theory also says that the color of light is dependent on the size of the corpuscles. Some of the main drawbacks of this theory are 1) This theory could not explain the phenomena of interference, diffraction, and polarization of light etc. 2) According to this theory the velocity of light in denser medium is greater than the velocity of light in rarer medium but this is proved wrong later 3) This theory assumes that the source of light looses the mass as it emits corpuscles; but not such determent in mass of the source of light is detected. 4) This theory proposes that velocity of the corpuscles increases as the temperature of the source increases as the temperature increases experiments have proved that the velocity of light is independent of temperature. Electromagnetic The electric and magnetic fields in a electro magnetic wave are continuously varying with respect to time and space. At any instant electric and magnetic fields are perpendicular to each other and also perpendicular to the direction of light. The electro magnetic wave is a transverse wave. At every point in the wave at a given instant of time the electric and magnetic field strengths are equal. The velocity of propagation of electro magnetic wave depends on the electric and magnetic properties of the medium. The main drawback of this theory is it failed to explain the photo electic effect and Compton effect. Electro magnetic theory also failed in explaining the black body radiation Quantum theories According to this theory, light energy is released from source discretely in the form of energy packets of specific frequencies called photons or quanta. Photons are propagated as waves and if necessary interact with matter as particles. This phenomena of Compton effect, photoelectric effect and black body radiation. The main drawback of this theory is it could not explain how it is connected with wave nature of light. Ordinary light If ordinary light and not polarized light is desired, both prisms may be withdrawn from the axis of the instrument; if the polarizer only is inserted the light transmitted is plane polarized; with both prisms in position the slide is viewed in cross-polarized light, also known as "crossed nicols." A microscopic rock-section in ordinary light, if a suitable magnification (e.g. around 30x) be employed, is seen to consist of grains or crystals varying in color, size, and shape. Plane polarized light Sunlight and almost every other form of natural and artificial illumination produces light waves whose electric field vectors vibrate in all planes that are perpendicular with respect to the direction of propagation. If the electric field vectors are restricted to a single plane by filtration of the beam with specialized materials, then the light is referred to as plane or linearly polarized with respect to the direction of propagation, and all waves vibrating in a single plane are termed plane parallel or plane-polarized. The human eye lacks the ability to distinguish between randomly oriented and polarized light, and plane-polarized light can only be detected through an intensity or color effect, for example, by reduced glare when wearing polarized sun glasses. In effect, humans cannot differentiate between the high contrast real images observed in a polarized light microscope and identical images of the same specimens captured digitally (or on film), and then projected onto a screen with light that is not polarized. The basic concept of polarized light is illustrated in for a non-polarized beam of light incident on two linear polarizers. Electric field vectors are depicted in the incident light beam as sinusoidal waves vibrating in all directions (360 degrees; although only six waves, spaced at 60-degree intervals, are included in the figure). In reality, the incident light electric field vectors are vibrating perpendicular to the direction of propagation with an equal distribution in all planes before encountering the first polarizer. The polarizers illustrated in are actually filters containing long-chain polymer molecules that are oriented in a single direction. Only the incident light that is vibrating in the same plane as the oriented polymer molecules is absorbed, while light vibrating at right angles to the polymer plane is passed through the first polarizing filter. The polarizing direction of the first polarizer is oriented vertically to the incident beam so it will pass only the waves having vertical electric field vectors. The wave passing through the first polarizer is subsequently blocked by the second polarizer, because this polarizer is oriented horizontally with respect to the electric field vector in the light wave. The concept of using two polarizers oriented at right angles with respect to each other is commonly termed crossed polarization and is fundamental to the concept of polarized light microscopy. Refractive index and its determination Refractive index, also called index of refraction, measure of the bending of a ray of light when passing from one medium into another. If i is the angle of incidence of a ray in vacuum (angle between the incoming ray and the perpendicular to the surface of a medium, called the normal) and r is the angle of refraction (angle between the ray in the medium and the normal), the refractive index n is defined as the ratio of the sine of the angle of incidence to the sine of the angle of refraction; i.e., n = sin i / sin r. Refractive index is also equal to the velocity of light c of a given wavelength in empty space divided by its velocity v in a substance, or n = c/v. Some typical refractive indices for yellow light (wavelength equal to 589 nanometres [10−9 metre]) are the following: air, 1.0003; water, 1.333; crown glass, 1.517; dense flint glass, 1.655; and diamond, 2.417. The variation of refractive index with wavelength is the source of chromatic aberration in lenses. The refractive index of X-rays is slightly less than 1.0, which means that an X-ray entering a piece of glass from air will be bent away from the normal, unlike a ray of light, which will be bent toward the normal. The equation n = c/v in this case indicates, correctly, that the velocity of X-rays in glass and in other materials is greater than its velocity in empty space. Relife method How does the mineral stand out compared to those surrounding it? Use high, medium, low relief to describe it. NOTE: Try to find a grain that is similar to the circled minerals are at the edge of the slide and in contact with epoxy, which has an index of refraction of 1.54-1.55. You can measure relief against this. Relief is low, moderate, high relative to n=1.54. Positive / negative can only be determined by looking your mineral up (+n>, -n<1.54) or by using the Becke line method Becke line The Becke line test is a technique in optical mineralogy that helps determine the relative refractive index of two materials. It is done by lowering the stage (increasing the focal distance) of the petrographic microscope and observing which direction the light appears to move. This movement will always go into the material of higher refractive index. This index is determined by comparing two minerals directly, or comparing a mineral to a reference material such as Canada Balsam or an oil of known refractive index (oil immersion). In microscopy, a method of testing the relative refraction indices. A bright line separates substances of different refractive indices. A faceted, transparent stone is immersed in a liquid of a known refractive index and viewed through the microscope. The faceted edges of the stone travel from light to dark, when focusing down, from the liquid into the gem, hence the refractive index of the stone is higher than that of the liquid, and inverted. Mainly suitable for small fragments. Beck line, Wild method. Open image in new window This method is standard procedure for mineralogists and petrographers and is designed for use with a petrographic microscope (Figure 4-6)*. A Becke line is a thin bright line at the interface between a transparent fragment and its immersion liquid when the relief is low (Figure 4-7). If relief is too high, the Becke line is hard to see and may appear as a bright shadow and, if the relief is almost zero, the Becke line may appear as colored dispersion fringes. We can eliminate color fringes and negate the effects of dispersion by using a monochromatic light source, usually sodium vapor light. Essentially all indices of refraction cited in articles and tables are values for the sodium wavelength (λ = 5890 Ǻ).

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