We are all familiar with sun glasses. Good ones use lenses that reduce glare and reflection by exploiting the properties of 'polarizing' materials. Loosely explained - what polarizing filters do is to confine light wave vibrations to certain directions : a plane! 

If we introduce one piece of polarizing material into our light cone, we effectively inhibit some of the light rays (waves?) so that the ones passing through are lined up in a specific [equal] plane. If we then introduce a second piece of polarizing material into our light cone and rotate it, when the second one is fully rotated 90 degrees with the first - all light will be obscured and prevented from passing through the second piece of material.  

We can use this knowledge and basic model of light to good effect in microscopy. If we place our first piece of material below the specimen slide and the second piece above it (rotated 90%), then something amazing happens with certain specimens. 

If the specimen contains processes and material that refracts and distorts light significantly enough to change its plane (direction of vibration), the light emerging from the specimen from these areas will shift plane relative to the other waves. Upon encountering the second piece of polarizing material (placed above the specimen) they will continue to pass through instead of being blocked. Remember - the second piece of polarizing material is aligned to block light travelling in a certain plane in the cone: any light not restricted to this plane will simply pass through! 

This technique is known as viewing specimens in 'cross-polars'. 

Many microscopes come with a polarizer and analyser built in. The 'analyser' being a reference to the top piece of material, and the polarizer being the lower (under the specimen) piece of material. They are often marked up as more expensive microscopes. 

 
Make your own Polarizer / Analyser 
Do you have an old pair of polarized sun-glasses? You do? Good because you can cut two pieces of material from it and use one under your slide and one above it. Try it! Rotate the top one until all light is extinguished. You need to find a slide which has something in it that interferes with polar light. Anything with crystals in it is ideal. A simple slide to use and test out this idea is one one that contains starch. Here's one I did showing starch in potatoes. 

Normal Illumination Cross Polar  Illumination

 
For a low cost, you can purchase a device and a set of slides ideal for cross polar viewing from the Mic-UK on-line shop at Onview. They've given me a small section there called Larry's resources - where you can purchase the things I recommend as good value for money and ideal for me projects here. The device is called a Polarspex. It is simply two pieces of polarizing material arranged 90% to each other but with provision to insert your slide easily between them. Its a good idea because it can be used with any microscope. 

This is what you would be purchasing should you wish to buy one instead of making your own:- 

 
Elastic bands are better than stage clips. They are gentler on the glass slide & allow for easy and precise repositioning of yer slide, give 'em a go! 
  

Here are some examples of slides with and without cross-polarization:- 

Benzoic Acid 

Normal Illumination Cross Polar  Illumination

Animal Hairs 

Normal Illumination Cross Polar  Illumination

Urea 

Normal Illumination Cross Polar  Illumination

 
In geology, the study of rocks and minerals often requires identification of material within the sample. A color wheel is used where individual gems, crystals, and minerals, can be matched according to their color and degree of polarization when observed under a polarizer / analyser pair. The technique of exploiting polarized light in Microscopical study is therefore a critical and essential one - not just a fancy way of putting a bit of color into an otherwise boring slide.  

There are other ways to interfere with that all-important cone of light. I have summarized some other techniques on the next page but really these two (dark-ground and cross-polar) are the basic ones most important to you! 

See other ways... 
  

Please note: I am intentionally using a mix of United States English spelling and UK English spelling in me (my) stuff here. I kinda (kind of) guess that if my Microscopy friends in the states can take the time to read me stuff and put up with me funny grammar, then I can at least try and use some american spelling on some words when I know them. One thing I like about American English is that many words containing the word part 'ise' (to polarise, to utilise) use the correct and true form of this - which is 'ize'. The word part is derived from the Greek word 'izein' : to employ; whereas the word - 'ise' is to flout etymology and logic! 

So now yer know. See yer get to learn not only microscopy 'ere (here) but a bit about words too :)) 

I digress. Sorry. Right onwards for one more page then... 
 

Mic-UK

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