POLARIZING MICROSCOPE

 POLARIZING MICROSCOPE

·       A Polarizing microscope is a type of microscope that uses polarized light to view specimens.

·       Polarizing microscopes are conventional microscopes with additional features that permit observation under Polarized light.

·       Although the invention of the polarizing microscope is typically attributed to David Brewster around 1815, Brewster clearly acknowledges the priority of Henry Fox Talbot, who published his work in 1834.

·       Polarizing microscope is commonly used to observe minerals, crystals, and other transparent or semi-transparent materials. It is also used to analyze the structure and properties of the above given materials.

Conversion of Unpolarized light to Polarized light

·       Light is an electromagnetic wave. Although light waves can vibrate in all directions, in general, they are described as vibrating in two directions at right angles to each other.

·       Any light which vibrates in more than one direction is called Unpolarized light.

·       Light wave that vibrates in a single direction is called Polarized light (The human eye is not sensitive to the direction of vibration of light).

·       Polarized light microscopes work by converting unpolarized light to polarized light. One way in which this can be achieved is by absorption of light vibrational movement in one specific direction. This can be done by certain natural minerals, including tourmaline, or by synthetic films that perform the same function.

·       Polaroid filters consist of tiny crystallites of Iodoquinine sulfate which are oriented in the same direction and embedded in a Polymeric filter. This embedding is done to prevent migration and change in the orientation of the crystals. The device which selects the plane-polarized from natural or unpolarized light is called a polarizer.

Parts of Polarizing microscope

i) Polarizing Filters

·       Polarizing filters are the most critical part of the Polarized light microscope.

·       There are usually two polarizing filters: a) Polarizer and b) Analyzer. 

·       Polarizer is located below the specimen stage and can be rotated through 360°.

·       Polarizer helps to polarize the light which falls on the specimen.

·       The analyzer is placed above the objective and may be rotatable in some cases.

·       Analyzer combines the different rays emerging from the specimen to generate the final image.

ii) Circular Rotating Stage

·       Circular Rotating Stage is the Specimen stage and it can rotate 360° to facilitate the correct orientation of the specimen with the Objective plane.

·       In several stages, a Vernier scale is also provided to provide an accuracy of 0.1° in the rotational angle of the stage.

iii) Strain – Free Objective

·       Any strain on the objective during installation can lead to a change in the optical properties of the lens which can reduce the performance.

·       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 assessed in order to ensure polarization and increased contrast.

·       Additionally, if the lens is attached too tightly on the frame, strain can be introduced. In addition, anti-reflection coatings and refractive characteristics must be precisely evaluated to guarantee polarisation and enhanced contrast.

iv) Strain – Free Condenser

·       Any strain on the objective during installation can lead to a change in the optical properties of the lens which can reduce the performance.

v) Coarse Focus Tension Adjustment

·       Coarse Focus Tension Adjustment is present in the bottom side for the adjustment and visualization of clear image of the specimen.

vi) Bertrand Lens

·       A specialised lens fitted within an intermediate tube or the observation tubes, a Bertrand lens brings an interference pattern created at the objective back focal plane into sharp focus at the microscope picture plane.

·       The lens is designed to permit simple observation of the objective rear focal plane, precise adjustment of the illuminating aperture diaphragm, and visualisation of interference patterns.

vii) Revolving Nose piece

·       As the stage and objectives of many polarising microscopes can rotate, a rotatable nosepiece is frequently included so that the specimen can be observed in the center of the field of vision even when the stage is rotated.

viii) Eye piece

·       The eyepieces of a polarized Light microscope have a cross-wire reticle to mark the center of the field of view.

·       Frequently, the cross-wire reticle is replaced by a Photomicrography reticle that aids in focusing the specimen and composing Photographs with a set of frames enclosing the view field to be shot digitally or on film.

Polarizing Microscope

Working Principle of Polarizing microscope

·       Light from the Sources pass through the Polarizer and forms the Plane Polarized – Light.

·       The Plane Polarized – Light enter into the Birefringent Specimen and Produces two rays. One is Extra ordinary ray (Blue) and another one is Ordinary ray (Red).

·       Both Extra ordinary ray and Ordinary ray combines and forms the Recombined Light Rays and results in the formation of the clear image of the specimen.

Applications of Polarizing microscope

·       Mineralogy

ü  To identify and analyze minerals.

ü  To study the crystal structure, refractive index, and other properties of minerals to identify.

·       Material science

ü  To study the structure and properties of transparent or semi-transparent materials.

ü  To analyze the microstructure of materials, such as fibers, films, and coatings.

ü  To study the effects of different processing techniques on the material.

·       Industrial inspection

ü  Used in industrial settings to inspect and analyze materials for quality control purposes.

ü  Used to detect defects, such as cracks, voids, and inclusions, in transparent or semi-transparent materials, such as plastics, ceramics, and glass.

·       Biology

ü  To study the structure and properties of biological specimens, such as cells, tissues, and organelles.

ü  To analyze the organization and orientation of molecules within cells.

ü  To study the effects of different treatments on biological samples.

·       Geology

ü  To study the structure and properties of rocks and minerals.

ü  To identify minerals, analyze the composition of rocks, and study the effects of different geological processes on the materials.

Advantages of Polarizing microscope

·       High contrast: Polarized light microscopes provide high contrast images of transparent or semi-transparent materials, making it easier to see small details and structures within the specimen.

·       Analysis of Birefringence: Polarized light microscopes can be used to analyze the birefringence of materials, which is the ability of a material to split light into two different beams that vibrate in different planes. This can provide important information about the structure and properties of the material.

·       Non-destructive analysis: Polarizing microscopes do not require any preparation or sample preparation, making them suitable for non-destructive analysis of materials.

·       Versatility: These microscopes can be used to study a wide range of materials, including minerals, crystals, fibers, films, and biological specimens.

·       High-precision rotating stage: The stage is spacious, pre-adjusted, and has 45° click stops. The stage’s smooth motion enables stable and simple rotation, resulting in great operability and high-quality polarised images. Because the stage is supported from the bottom near the optical axis and is equipped with steel cross roller guides, it is exceptionally stable and durable. The focus stroke has been increased to 30 mm, making it easier to observe tall samples. Clamp-type upper limit focusing mechanism makes sample exchange simple and secure.

Limitations of Polarizing microscope

·       Limited to Transparent or Semi-transparent materials: Polarizing microscopes are not effective at analyzing opaque materials, as they rely on the transmission of light through the specimen.

·       Complex to use: Polarizing microscopes can be more complex to use compared to other types of microscopes, as they require the use of polarizing filters and the correct orientation of these filters.

·       High cost: Polarizing microscopes can be more expensive compared to other types of microscopes, due to the specialized components and features they require.

·       Limited to transmitted light: These microscopes can only be used to study specimens in transmitted light, meaning that they cannot be used to observe surface features or structures.