Microscope polarising light

Microscopic Examination in Polarised Light.—This is carried out with a microscope furnished with a polarising apparatus, a plate of selenite about 0-2 tnm. in thickness being fitted on the diaphragm of the microscope and hence below the preparation. The magnification should be 150-170 diameters. 

The apparatus used for IR microscopy is a Fourier-transform infrared (FTIR) spectrometer coupled on-line with an optical microscope. The microscope serves to observe the sample in white light at significant magnification for the purpose of determining its morphology, as well as to select the area for analysis. The spectrometer, on the other hand, enables study of the sample by transmission or reflection measurement for the purpose of determining the chemical composition. It also provides information about the microstructure and optical properties (orientation) of the sample. It is possible to apply polarised light both in the observation of the sample and in spectrometric measurements. 

Most often, fragments of paint have a multilayer structure. Each layer (about 10-50 pm thick) is made up of paint material and is a mixture of many chemical compounds. Paint smears, on the other hand, are usually made up of one or two layers of paint material mixed with and sunk into the base (e.g. among fibres of the fabric). The morphology of the paint trace can be observed under an optical microscope. The number of layers visible on a cross-section of paint chip, their colour and thickness are characteristic for the coat of paint from which it originates (Fig. 11.1). The layers are often better visible if the sample is illuminated with polarised light or if fluorescence of sample is excited by illumination with UV light. 

Polarised l t microscopy, in which the sample is illuminated with Hnearly or circularly polarised light, either in a reflection or transmission mode. One polarising element, located below the stage of the microscope, converts the illumination to polarised light, while a second polariser is located between the objective and the ocular and is used to detect polarised light. Various characteristics of the specimen can be determined, including anisotropy, polarisation colours, birefringence, and polymorphism. 

Because different polymorphs have different internal structures, they will belong to different crystal systems therefore, polymorphs can be distinguished using polarised light and a microscope. The crystals can be either isotropic or anisotropic. In isotropic crystals, the velocity of light is the same in all directions, while anisotropic crystals have two or three different light velocities or refractive indices. In terms of crystal systems, only the cubic system is isotropic the other six are anisotropic. 

The analysis of materials that possibly contain asbestos can be carried out by various methods, however the dispersion staining microscopic technique is considered to be a quick, reliable and cost-effective method that is used by the majority of laboratories that routinely carry out this type of work. This method should be in accordance with MDHS 77 (1994), which is published by the UK Health and Safety Executive and forms part of the series of the methods for the determination of hazardous substances, titled Asbestos in Bulk Materials -sampling and identification by polarised light microscopy (PLM). 

RPE Respiratory protective equipment PLM Polarised light microscope... 

It is easy to observe spherulite growth in a thin film of low molecular weight polyethylene oxide, melt between a microscope slide and a cover slip, using polarised light microscopy. The spherulites grow as discs once their diameter exceeds the film thickness of about 0.1 mm. The discs have a radiating fibrous appearance and a Maltese cross pattern with arms parallel to the crossed polarising filters below and above the specimen (Fig. 3.24b). However, these two-dimensional spherulites are a rarity in nearly all cases the spherulites are three-dimensional with polyhedral boundaries. 

Low- and high-powered microscopes are used to examine the morphological features of the fibres and the initial determination of whether the fibre is natural or man-made. FTIR microscopy can be used on a synthetic fibre to provide information in relation to the functional groups present this can be used to pinpoint which synthetic fibre it is. Polarising light microscopy is used with synthetic fibres plane-polarised Ught interacts with the fibres in order to provide refractive index values (many of these fibres have two refractive indices due to the chemical structure of the fibre and are said to be birefringent). This helps in the identification of the synthetic fibre. 

---