Polarizing Optical Microscopy
Introduction Polarizing optical microscope (POM) is used to identify liquid crystal textures. The principle of the microscope, the polarization of light and recording of textures are elucidated.
Light is electromagnetic radiation. Electric vector and magnetic vectors are at right angles to each. Thus by aligning the electric vector different types of polarization viz., linear, circular, elliptical, can be deduced. A microscope magnifies the object. Combination of light polarization and magnification results in the polarizing optical microscope. It is extensively used in liquid crystal research, to observe liquid crystalline textures.
Light is an electromagnetic wave comprising of electric and magnetic vectors perpendicularly aligned to each other. This is first suggested by Maxwell in the 18th century, he proposed the light travels in discrete energy packets and named them as photons. Einstein explained the concept of quanta and summarized that energy possessed by each quantum is plank constant (h) times the frequency (γ). [E=nhV]. In fact, light energy is now seen as integral multiples of quanta.
Types of Polarization
The word polarization refers to the alignment of the electric vector of light with one another. There are three types of polarization viz. linear, circular and elliptical. If the tip of the electric vector rotates on a line it is referred to as linear polarization. Similarly, if the tip of electric vector rotates clockwise on the circumference of a circle it is right circular polarization. If it rotates anti-clockwise it is left circular polarization. If the tip of electric vector rotates clock wisely on the circumference of an ellipse it is right elliptical polarization and if it rotates anti-clockwise it is left elliptical polarization.
Magnification of a microscope
The product of eyepiece, an objective gives the magnification of a microscope. Thus keeping the objective fixed various eyepieces will be changed. They are labelled as 5x, 10x etc where 5x refers to five times magnification of the image. The purpose of the microscope is to magnify images.
Liquid crystal materials (LCM) are organic compounds which exhibit the properties of solids (lattice) and liquids (viscosity). At low-temperature LCM Aer crystals and the high temperatures, LCM are liquids. Between low and high-temperature states they exhibit liquid crystalline behaviour.
Polarizer and analyzers
Polarizer and analyzers are two sheets which comprise of long-chained organic material aligned parallel to each other. So in a polarizer optical microscope, the liquid crystal is sand witched between the polarizer and analyzer. Initially, the polarizer and analyzer are crossed, i.e. they are kept at right angles to each other so that no light can pass through them. This condition is referred to as a crossed condition.
Identification of Liquid crystallinity through POM
A typical POM comprises a polychromatic light source, polarizer and analyzer. The objective is fixed whereas the eyepiece is changed to the desired magnification. Polarizer and analyzers are crossed and hence no light is visible. When a light crystal is introduced, liquid crystal rotates the plane of polarization and hence texture is seen in the microscope. Thus through this microscope, liquid crystallinity can be observed. To test whether a sample exhibits liquid crystallinity, it is sandwiched between polarizer and analyzer and mounted in the optic path of the microscope if the sample exhibits mesogenic nature a texture is observed.
Automation of POM
In the first versions of POM, the eyepiece is the only way to note the changes in the sample. By human eye, the changes are observed physically. In the later versions, recording, retrieval and storing features are enabled. The image seen through the eyepiece is stored through a digital camera and later stored in the desired place. Analog to digital and digital to analogue converters are used. A high-resolution pixel camera is employed to record the textures. Special software’s are developed to accomplish the recording procedures. Still photos and movies can be recorded with different pixel resolutions. The photos can be stored in any desired format (JPEG, IMG, etc).
Advancement in POM
Most of the commercially available POM accommodates the mounting of hot and cold stages to monitor the temperature. Rotation of the disc is also facilitated. Optical filters of various frequencies are also appended with the instrument.
Precautions to be followed
The following precautions are to be followed while using the POM
1. The optic axis of the instrument should never be disturbed.
2. Eyepieces should be maintained and cleaned properly guarding them against fungus formation.
3. Before using polarizer and analyzer should always be in the crossed position
Nature along with different polarization of light are discussed. Polarization optical microscope working and advancements are deliberated.