Polarizing Microscopy: Functions and Uses
Although extremely valuable as an examination and investigative tool, polarizing light microscopy is still not as utilized as other types of microscopy. It has all the functions and features of standard brightfield microscopy but it offers plenty of visual details and information that some microscopy techniques can’t always provide.
Unlike other types of optical microscopy, polarizing microscopy can provide information about boundaries and colors in minerals and thin sections of rock samples which exhibit different refractive indices. It can also offer excellent visuals of both isotropic and anisotropic specimens, allowing the observer to distinguish the two. Because of its optical capability, polarizing microscopy can provide composition and structure information about specimens which are necessary for diagnostic and identification purposes of the samples.
How polarizing microscopy works
Although light travels in a straight line, it is in the form of a wave or as electro-magnetic vibrations. Non-polarized light, such as light coming from the sun, vibrates in different random directions. Polarized light, on the other hand, travels in vibrations that have direction, whether it’s circular, elliptical, vertical or horizontal. Polarization of light can occur through manipulation using different sets of filters.
The direction of the vibration is transverse or oblique to the direction in which it is spread. This type of light is referred to as plane polarized since all the vibrations occur in only one plane – that is, only one vibration direction occurs. Light that comes from the sun is unpolarized but once it bounces off a surface, it becomes semi-polarised. To the human eye, which is not sensitive to the directions of vibrations of light, plane-polarized light will be difficult to detect. Its effect can only be distinguished when its color or intensity changes, at which point it affects the eyes. Wearing polarized sunglasses, for example, can significantly reduce glare, something that the wearer can become aware of.
In a polarizing microscope, two filters are used, both of which lie within the optical path. These are the polarizer filter and the analyzer filter. Their permitted directions of vibration are positioned at right angles. The polarizer’s vibration direction is usually fixed in the EW (East-West) direction and can be fixed left to right. In most polarizing microscopes, the polarizer can be rotated a full 360 degrees. This is often found under the specimen stage. The analyzer, on the other hand, is aligned in the NS (North-South) direction and is also often rotatable in most polarizing microscope units. It can be manipulated to move in and out of the path of light if necessary. The analyzer is found on top of the objectives.
When used together, the polarizer and analyzer can generate detailed contrast in many specimen images, able to produce aspects in many materials that are not often achievable with other types of microscopy.
How polarization occurs in a polarizing microscopy
Polarization colors are the result of the interference of lightwaves that pass through the material, usually an anisotropic specimen. When light passes through the material, its components are split into two. To visualize this more effectively, imagine white light without the colors that interfere with the visuals destructively.
These two light components travel at varying speeds when passing through the material, which results to light having dissimilar refractive indices, called refringences. The numerical difference between two or rays of light of refringences is called birefringence. In this case, only wavelength becomes the variable and phase differences will vary depending on the different wavelengths. This is the reason why certain colors will appear more distinct in contrast with others. When an observer looks at a specimen through a polarizing microscope, a colored image will appear because certain wavelengths have been removed.
The maximum color of a sample and its extinction angle (usually an anistrophic material) can be achieved and observed. This is why kaleidoscopic colors are often seen in images of minerals and crystals as a result of crossed polarizers used in a polarizing microscope. Extinction angles also help observers distinguish one material from another.
Uses of polarizing microscopy
Both transmitted and reflected light can be used with polarizing microscopy. Although polarizing microscopy is better known for its application in the field of geology, it can also be utilized as a tool for the study of numerous materials. Aside from minerals and rocks, it can also be used with natural, synthetic, industrial, manufactured, extracted and refined elements and composites. These can include anything from ceramics, fibers, polymers, cements and crystalline substances to other biological materials such as wood, urea and DNA. As such, polarizing microscopy is also an excellent tool for biology, metallurgy, chemistry and medicine.
Polarizing microscopy is an extremely useful method to obtain a wealth of information about the structure (usually 3-dimentsional) and composition of a wide variety of materials and samples. Not only that, it is also an accessible investigative tool, relatively inexpensive and very reliable, able to provide detailed information that is not often available with other microscopy techniques.
