Polarising microscope

POLARISING MICROSCOPE

1. 1 Polarizing Microscope DEPARTMENT OF GEOLOGY Guided By :- Presented By :- Dr. Narendra Joshi Amit Rathore Class- M.Sc. ( I Sem)

2. CONTENT:- INTRODUCTION PARTS OF POLARISING MICROSCOPE WORKING OF POLARISING MICROSCOPE SIGNIFICANCE CONCLUSION REFRENCE 2

3. I NTRODUCTION : - Polarizing Microscope( Petrological microscope): In the year (1768-1851), William Nicol discovered polarizing Microscope. It is used in the study mainly for the identification of optical properties of the given specimen ( i.e. rock ) which is in the form of thin sections of the rocks and minerals. It is a typical transmitted-light microscope. 3

4. 4 Fig no 1 Monocular polarized Microscope

5. 5 Microscope:- A microscope is an instrument which is used to study the micro or too small objects which is not sufficient to see clear objects by our naked eyes. There are many different types of microscope which is used on the basis of essential field work. It is majorly three types of microscope:- a) Simple Microscope b) Compound Microscope c) Electron Microscope But In The field of Geology, we have to study the thin section of the rock type, the type of modified Compound Microscope ( i.e. Polarizing Microscope) is used.

6. 6 Polaroid is an optical filter which is filtered the unpolarised light( i.e. light source of bulb, natural light, etc.). It is a device which is used to analyze or create polarized light . Normal light vibrate in many plane or vibration. In polarizing Microscope, a polarizer is used which allow passage of light of one vibration direction. Polaroid:- Fig No 2 Fig No 3

7. Parts of Polarising Microscope It is majorly different parts of this Microscope on the basis of Mechanical and Optical Parts. Types of Petrological Microscope Optical Parts Mechanical Parts Polarizing Prism Types of Polarizer prism Bertrand lens The Objective lens The Condensing lens Ocular/ Eyepiece Huygens eyepiece Reflector Circular rotating Stage Sub-stage Verniers Scale Tube Objective Holder Slot of Accessories Base Fine and Coarse Focus 7

8. On the basis of Mechanical Parts Rotating Stage :- The thin section of a mineral to be examined is placed on this stage. The axis of rotation of the stage coincides with the microscope axis. This is also called as Graduate stage. 8 Fig No. 4 Fig No. 5

9. 9 Sub-stage :- The sub-stage is the ring beneath the revolving stage. It carries the polarizer and, in some microscope, the condensing system. It is described below in connection with the condensing system. Fig No. 6

10. 10 Verniers :- Attached to the sides of the stage of most microscopes, there are certain auxilary scales called verniers after the inventor, Pierre Vernier (1631). These verniers may longer or a little shorter than the divisions of the main scale. Fig No. 7 Fig No. 8 Verniers Scale

11. 11 Objective Holders :- Objective may be attached to the tube in one of four ways. In old microscopes it is necessary to screw each objective into place as it is needed. Strain can also be introduced if the lens is mounted too tightly on the frame. Also anti reflection coatings and refractive properties must be accurately accessed in order to ensure polarization. Fig No 9

12. 12 Observation Tube :- The tube is the hollow cylinder to which most of the optical parts are attached. In most microscopes it is straight or it is inclined at an angle to enable the observer to sit at ease and work .

13. 13 Base:- This base is specially designed to balanced and proper stand and its shape is like Horse -shoe shaped base .

14. Slot of Accessories :- In old microscopes connecting the tubes with the objective is a short collar containing a slot for insertion a Quartz wedge or a gypsum or a mica plate . 14 Fig No 10

15. 15 Fine and Course Focus Adjustment :- The large and small circular adjustments on the lower portion of the body of the microscope are used to rack the stage up and down for changing the focus of the objective lens in use . This adjustment should be used to focus the thin section.

16. On the basis of Optical Parts Polarising Prism :- Many of the most important characteristics of crystals are determined by Polarised light. For obtaining polarised light some form of calcite (Iceland Spar) prism is used. Recently in less costly Polaroids are used for obtaining polarised light. It is basically of two types:- Polariser :- Analyser :- a) Polariser:- The polariser is fitted below the stage. It transmits polarised light in the East-West direction. 16

17. b) Analyser :- The analyser is placed in the microscope tube above the stage and is removable. The analyser transmits light vibrating in the North-South direction. When both the polariser and analyser are in position, they are said to be crossed. Both positions are in the right angle ( i.e. perpendicular to each other). While such condition happen then normally no light will pass through and the viewer will only see darkness. 17 Fig No. 11

18. Huygens Eyepiece :- The ocular of a microscope is not nearly so complicated as objective. In most forms but two lens are used. This Eyepiece is the one most commonly used in petrographic microscope. It is achromatic in the sense that images of different colour appear of the same size. 18 Huygens Eyepiece Ramsden Eyepiece Fig No 12 Fig No.13

19. Bertrand Lens :- . This is given by mineralogist Emile Bertrand. This lens is used to observe interference figures. A specialized lens mounted in an intermediate tube or within the observation tubes, a Bertrand lens projects an interference pattern formed at the objective rear focal plane into focus at the microscope image plane. The lens is designed to enabled easy examination of the objective rear focal plane, to allow accurate adjustment of the illuminating aperture diaphragm and to view interference figures. 19 Fig No. 14

20. 20 Ocular/ Eyepiece :- This is the lens that you look through at the top of the microscope. The eyepiece of the microscope carries cross-hairs, one in the North-South direction and the other in the East-West direction. These cross-hairs help in locating a particular mineral grain for detailed examination. They are also useful in aligning cleavage fragments for making angular measurements. Fig No. 15

21. 21 Condensing Lens :- The condensing lenses are required below the stage . They are used with high power objective and produce converging light rays when required. It is located with lower analyzer. A polarizer condenser has following 3 characteristics- Built-in rotatable Polarizer. Top lens-out construction, when parallel light illumination at low magnification is required. Strain-free optical system, like the objectives. Fig No . 16

22. 22 Iris Diaphragm :- This diaphragm is located below the stage. It regulates the intensity of light . This is an adjustment nearest the light source that allows more or less light to be transmitted to the stage. Fig No. 17

23. 23 Reflector:- This is the reflector part of the microscope which is situated in the bottom of the instrument. It is made up of Plano-concave mirror . Fig No. 18

24. Working of Polarising Microscope:- Preparation of thin section- A thin section of a mineral (or a rock) is made by grinding and polishing on one side of a piece of it. This side is then cemented to a slip of glass with canada balsam. The mineral chip is then ground down on the other side with successively fine grades of carborundum powder. When a suitable thickness is left ( about 0.0265-0.035mm) a cover slip is cemented on it. 24

25. Pathway of Light 25 Fig No . 19 Fig No. 20

26. 26 Behavior of transmission light on the following conditions :- When an specimen is not present in the microscope – Analyzer is pushed completely in place . Now Polarizer is placed and is rotated until the field of view become dark. When an specimen is in Anisotropic material – Light passing through these substances splits into two rays that vibrate in perpendicular direction, Where transmission of light under crossed polarizer ( light splitting ). Pleochroism possible. When an specimen is in isotropic material- No transmission of Light under crossed polarizer. Materials whose refractive index not depend on the direction that light travels are called Isotropic materials . Fig No. 21 Fig No 22

27. 27 Fig No. 23

28. Conclusions :- Microscope dedicated for use with polarized light are very sophisticated instruments having components specifically designed to minimized strain and provide sharp, crisp and clear images of birefringent specimens. For simple qualitative work , a standard microscope can be converted for polarized light studies. Typically, a small circle of Polaroid film is introduced into the filter tray or beneath the sub-stage condenser and a second piece is fitted in a cap above the eyepiece or within the housing where the observation tubes connect to the microscope body. 28

29. SIGNIFICANCE:- It is used to identification of minerals in research lab. It is used to determined of rock type in the geological work. It is used to recognized of crystallization sequence . It is useful for mineral exploration. Document Deformation History . It is used to collect all different optical properties 29

30. 30 References:- a. Ore Microscopy by John Wiley b. Optical Mineralogy by Paul.F.Kerr c. Olympus Life Science by Internet

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