GENERAL CHARACTERISTICS OF LIGHTElectromagnetic spectrum include Gamma rays, X-rays, UV rays, visible light, infrared and radiowave. The light is considered as waves of electromagnetic energy vibrating through the medium of other. The light is' a combination of seven different colours ranging in wave length from 700 m/u. 'for red and 410 m/u for violet (1 m/u = millionth of a millimeter). The light is visible because the waves of these lengths are capable of stimulating our optic nerves. In the transmission of light energy is passed on from one particle to the next, and so on. The Transmission takes place in the form of waves. There are motions in two directions. 1) In the direction of propagation there is an uniform rectilinear motion. 2) In the transverse direction the motion seems to take place around a circle with uniform velocity and in a straight line as a to and fro motion with periodic, variation of velocity. It moves from its original position with decreasing velocity. After reaching a point of maximum displacement it returns with increasing; velocity to its original position, Again it moves with decreasing velocity until it reaches a displacement equal and opposite in direction and' returns. The motion is called periodic motion or vibration or harmonic motion. A combination of these two motions', harmonic and rectilinear, prevails in the form of a curve wave form. This curve is celled a simple harmonic curve.All the horizontal sections of the above indicatrlcea will be circular in owfcltae and at: A is clear that they will be isotropic under crossed nicolsi TW normal to these sections coincide with the ' 0' axis. This &'reet.ton i taOvr as optic ggcig direction. Optic axis direction as one in wb.icVi there 3-s no double refraction. All the other sections are elliptical at\k so sn isotropic. As there is only one optic axis is tetragonal and hexagonal minerals they are called uniaxial. The biaxial groun includes the minerals of the orthorhombic monoclinic and triclinic systems. In the biaxial minerals thure are three especially important/vibration directions. There is at right angles .to each .other. They are designated as X, Y, and Z. These directions are sometimes called axes of elasticity orether axes. . The light vibrating parallel to 'X.' has themaximumvelocity. Its refractive index is light vibrating parallel to.' has the intermediate velocity. Its refractive index is refractive Index is The optical structure, Fletcher's indie.-vi: :lx , is a triaxial ellipsoid. It has three axes ax right angles to each other. The lengths of the axes are proportional in length to the indices ana"- This figure exhibits the optical symmetry. The velocity andplace of vibration of any light 2355s ray traversing the crystal may be derived from this figure. Any section through the indicatrix will be an elliptical section. - The major and minor diameters of such a section vdll give the indices and- planes of vibration of the two possible rays. The wave fronts lie in this plane. There are two circular sections. The radius of these circular sections is the semiaxial length of these sections is isotropic. Light passing in the direction normal to these directions suffers no double refraction. These two are the optic axes directions. These optic axes lie in the Z-Z plane and so this plane is called optic axial ulane. 'Y is normal to this plane an d it is called Q-ptjrtTnormal. The acute angle between the two axes is called psftic axial angle or '2V. The ether axis -which bisects this angle is called acute bisectrix. When ' Z' is the acute bisectrix the mineral is negative.She -grffiifo,. ellipsoid is vibration, velocity ellipsoid. The three principal axes are proportional to the velocities of light vibrating parallel to X, Y svd S respectively. In .this ellipsoid there sire two c;..'.vrai r fiactions . In the directions normal to these eirovJ.ir sections light err not be doubly refracted. These are the optic .wis directions. These directions are slightly different from the cirections of optic axes of the indicatrix (-primary optic t exes). These are called secondary o-ptic axes for the sake, of distinction. The angular variation between, the two sets of optic axes .is usually very small, perhaps 1 or less. In the figure the circle with radius 1/ interseots the ellipse whose major and minor axes are 1/ and 1/ in the points S', S, S' S' In these directions (secondary optic axes) velocity is same for both rays. Hence within the crystal these rays travel together without double fraction. However, there is no common wave front for these two rays (Tangents mm and nn). Therefore on emerging they dp suffer double refraction. In fact two external light waves are formed. The directions are Su and Sy, Therefore these secondary optic axes directions are of minor importance. Whenever optic axes are spoken of, they are always ihe primary optic axes. The primary optic axes (PP, P''$>) directions are normal to the tangent planes (tt, t't') the direction of the external wave is given by the normal, P-W.PETAPLOGIOAL MICROSCOPE AND ITS PARTSThe microscope used for the investigation of minerals is called the Petrological or Pe trogra ph'ic microscope. The PePrologica 1 microscope is a compound microscope. Pig A petrographic microscope essentially consists of a stand having a 'heavy foot and a pillar. A revolving stage and a tube are attached to this pillar. A mirror and a polariser are found below .the stage in most microscopes. The tube carries the objective, the analyzer and an occulgr or eye piece.The stage is a circular table upon which the thin section is placed for examination. The stage may be rotated. The amount of rotation is determined by means of a graduated scale and vernier. It has a central opening through which the light passes. The thin section is usually held in place on this stage by spring object clips. . As the vernier is made to coincide with successive divisions of the larger scale, each successive coincidence indicates an advance of 1/10 of a division.The tube is straight or inclined. Objective lens is attached to the lower end of the tube. There is -a short collar near the objective holder. This collar contains a slot for the insertion of optical accessories like quartz wedge, mica plate or gypsum plate. 'The centre 'of rotation of the stage should be on the axis of the tube. This centre should coincide with the intersection of the cross hairs of the ocular. If the centre of rotation help or two appears lying outside this intersection, it is set right with the/ centering screws. The focusing is d one "by ra is ing or lowering the tube. In general there are two adjustments, one coarse and one fine. In some recent, microscopes the stage is adjusted. The substage is ring beneath the revolving stage. It carries the polariser and the condensing lens.One djaphram is found above or below the polarizer. It is used: to regulate the amount of light admitted to the eye. Another diaphram is placed above or below the Bertrand lens. It is used to cut out interfering minerals from the field of view. Usually a diaphram is of the iris type. This consists of a number of overlaping leave attached by pins to the rim of a casing. The central opening of this is enlarged or diminished by moving a lever.The illuminating apparatus of a microscope consists of a mirror, a diaphram and' a system of condensing lenses. By means of a lever the condenstag lens may be thrown out or kept in. The mirror is used to reflect the light from the source to the object. A plane mirror forms one side and a concave mirror on the other. The mirror may be inclined in any direction. The plane mirror is used with low magnifications and the concave for higher magnification. Rico 1 prism or polaroid is used to produce polarised light. It forms an important adjunct to the petrographic microscope. There are two-polarizing prisms. The lower nicol is called the polarizer and the uppernicol analyzer. The upper nicol is made to slide in a slot in the tube. These two nicols are positioned in such a manner that their vibration directions are perpendicular each other. When they are in exact crossed position the field appears dark. Bsrtrend lens swings to and fro in a lever in the upper part of the tube. This lens acts as a small microscope. In connection with the ocular /magnifies the image. The objective lens at the lower end of the tube receives. The light from the object first. It magnifies the image. An objective is of low power when its focal length is 13mm and its magnification is less than Yo diameters. It is of high power when its magnification is over 40 diameters. In between these two is of medium power. In most forms of the eyepiece or ocular two lens are used. The Huygens or ^negative eyepiece consists of two simple plane convex lenses. The plane surfaces are found toward the eye. The upper' lens';is called eye lens and lower collective or field lens. This is commonly used. Fig. In the Ransden or positive eyepiece the convex sides of the two lenses are facing each other. This is used in special work such as making measurement with micro meter.There are two cross hairs in the ocular, perpendicular to each other and parallel to the vibration directions of the nicols. Apart from the above parts there are some optical accessories like gypsum plate. Quartz wedge. mica plate and bere compensator.OPTICAL ACCESSORIESThere are mainly 4 optical accessories employed in the study of minerals in thin section. They are 1) Gypsum plate 2) micaplate 3) Quartz wedge and 4) Berek compensator. A Gypsum paste is cut to such a thickness as to produce a violet Interferenoe colour (colour value about.570) of the upper limit of the first order between crossed nicols. It is usually cementedjbetween narrow glass plates and mounted in a metals holder. It is marked sensitive, tint, Red First order or Gyps rot I. The arrow at one endjaf 'this acjDessorx indicates the direction of vibration of the slow ray, %. It is called a sensitive tint because the eye is especially sensitive even to a slight change of the tint in this part of the colour scale. Therefore the gypsum plate is used in the case of minerals of weak double refraction aid especially with minerals which produce gray interference colours. Fig.A micaplate is prepared with a thin flake of white mica. It is so thin a_s_tp;produce a J5a_le_gray interference colour (colour; value\ about 150.)''. The fast ray would gain one quarter (1/4/S) of a wave length in phase over the other. This thin mica flake is usually mounted between long.and narrow glass plates. . It is commonly marked 1/4M with an arrow indicating the Z, slow opt ic al d irec ti on. The mica plate of this small retardation is especially Valuable^for. Testing minerals which give bright interferencet colours of II ana III orders. Fig. The quartz wedge consists simply of a very thin tapering wedge. The wedge may be cut so that its length is parallel either to 'c ' | or 'a'. Crystallographic axis of the crystal quartz. This wedge is cemented to a piece of glass that is mounted in ametal holder Fig. The plate is generally marked with the letter Q (quartz) and an arrow. When the wedge, is cut with its longer direction at right angles to the vertical axis of a quartz crystal the arrow is marked' Z. (or_y) , the slow ray direction. When it is cut with its longer direction parallel to the vertical axis of the quartz the arrow is marked X (orcC ) the fast direction. The quartz wdge furnishes a prismatic section of varying thickness aril, of known orientation, when the quartz wedge is pushed, into the slot in the microscope at 45 position between crossed nicols, interference colours occur in the following order as the thickness of the quartz wedge Whereases: violet, bj,ue, green, yellow, orange, red. This sequence of colours is repeated three times quite distinctly and then the colours becomes fainter and not so cltfar.\;...This series of interference colours is divided into orders as shown hei-ow in the figure.USES OF THE MICROSCOPE ACCESSORIES:To determine the exact extinction' position of any mineral the mineral section is brought to^tne. Posit ion of maximum dark-ness between crossed nicols. The#the gypsum plate is inserted through the slot in the microscope at 45 position. If, it is the exact extinction position of the mineral the entire field including the mineral is coloured, in violet. Even if there is a slight difference in position the mineral shows a markedly different colour. There the. siage with the mineral may be slightly rotated to the position in. which the entire field uniformly shows violet colour.2) Determination of relative velocities and optic ai^n of minerals; the two rays coming. To determine the relative velocities of from a doubly refracting' mineral section, the section is brought tp the 4 5 position under crossed nicols and the interference coloured is noticed. How an accessory plate is inserted. If the like vibration directions ((i.e.) fast of the mineral and. fast of the accessory and slow of the mineral and slow of the accessory) coincide the retardation increases and the intensity of the interference colour also increases. When unlike vibration directions' coincide the colour fall* as the retardation deoreaoes. Irom these results, relative velocities may be determined easily as the accessory of known vibration direction is employed. Uniaxial crystals or cleavage fragments that show consistant elongation must be elongated parallel to 'c '. If the fast ray is parallel to the direction of elongation the mineral is said to have negative elongation (ie) Length-Fast. If the slow ray is parallel to the direction of elongation then the mineral is said to have positive elongation (i.e.) Length-slow. Pig. Thus the c crystallographic direction of the uniaxial mineral may be determined with the help of the habit and cleavage and the sign of the uniaxial mineral may be determined.Along the c direction extraordinary ray; vibrates. If it is fast the mineral is negative and if it is slow the mineral is positive. Determination of optic sifen from uniaxial optic^axls interferencef iguire: To determine, the optic sign from uniaxial" 6ptie a*16 figure an appropriate accessor)'plate must be chosen and used. If the bireferingence of the mineral under examination is weak and the interference figure is devoid of colour bands, it is best to use a gypsum plate. Strong bireferingence 8nd bright colpurg in the interference figure call for use of a mica plate. The quartz wedge is best, used when ari interference figure shows many isoehromes or colour bands'. In the uniaxial optic axis interference figure) "Sfee vibration planes are radial and tangential in relation to the cone of rays. The radial vibrations are in the planes which pass through the optic axis and so those of extraordinary rays (e). Tangential vibration planes correspond to ordinary rays (o).j=i