What is polarized light microscopy?
Polarized light microscopy is a magnifying technique very similar to a basic light microscope that you will find in a school laboratory, however instead of normal light waves, polarized light waves are used.
What is polarized light?
Normal light waves oscillate (vibrate) in random directions, polarized light waves oscillate only in one direction. Unpolarised light can be polarized with a filter which allows light oscillating in only one direction to pass through (this is what polarized sunglasses do).
How does it work?
A polarizing filter is placed over a light source which is shined onto the object you want to view. When this polarised light hits the object, it will either pass through (appearing dark), or refract (bend) into a new direction. An ‘analyzing’ filter then recombines the refracted light waves. When they recombine, the waves interfere with one another. The way the light interferes after passing through a sample can be used to identify what it is. Only anisotropic materials will refract light, so not everything can be seen with this technique.
What is an anisotropic material?
Anisotropic materials have properties which are direction-dependent. A good example of anisotropy is a plank of wood: wood splits more easily along the grain than across it due to the way the atoms are ordered. The opposite is isotropic, where properties are the same in all directions (most transparent liquids).
Imagine a droplet of water. Rotating this droplet does not change its optical behaviour, light refracts (bends) the same way as it passes through regardless of direction, it has one refractive index. In an anisotropic material, like a ruby crystal, its optical behaviour changes when you rotate it. In one direction light may travel through, in another direction the light splits into two different paths. As it has two refractive indexes, it’s called birefringent.
(Left) Elle demonstrates the 'bend and refract'. Light splits into two as it passes through her body and bends into different paths. This is double refraction caused by a birefringent material (Elle in this case). Birefringent materials create beautiful images like this of dopamine (right) captured by microscopy wizard Ingrid Parrington.
What makes the images so colourful?
Different colours are different wavelengths of light. Anisotropic materials interfere with the polarized light waves, and this interference leads to some wavelengths, and thus colours, getting cancelled out.
What makes it interesting?
New advances in this field have improved the resolution to be able to image cells and tissues that are weakly birefringent due to the way the molecules are organised. This could help in the diagnosis of disease. Here is an example where polarized light microscopy is used to detect collagen (multicoloured strands) in human breast tissue. The way collagen is organised can reflect healthy or diseased tissue, and in this case highlights breast cancer.
When is it used?
Asides from its medical and biological uses, geologists often use this microscopy tool to find out what type of mineral they are looking at, as crystals are commonly birefringent materials.
Where can I find out more?
Scientists: click here
Non-scientists: click here