My previous post described how I use the technique of Cross Polarization to create colorful images of birefringent crystals.  This technique reveals details of the underlying structure of the crystal itself.  It also effectively color codes different parts of the crystal.  These patterns and colors not only contain useful information for scientists who study their properties, they also reveal the beauty of the natural world around us.

The color of the light passing through these crystals can be further enhanced by placing a Full Wave Plate between the first polarizer and the sample.  A Wave Plate is an optical component, in this case a piece of specially designed plastic, which changes the polarization of light passing through it.  How much the polarization changes depends on the color of the light.  If you read my first post this will sound very similar to the birefringent effect of the crystals.  Indeed the physics is the same.  What is different is that a wave plate is carefully designed to produce a very specific change to the polarization of the light which passes through it.

Trying to explain what a full wave plate does to the light passing through it is well beyond the scope of this post.  Its effect however is that can be used to ensure that certain very specific colors of light will end up being blocked by the second polarizer in the cross polarization technique.  White light is what our eyes see when all colors of light are present.  When one or more colors are blocked from reaching our eyes, the result is we see the complement of the blocked colors.

Putting aside the physics of electro-magnetic waves and color theory, the aesthetic effect of adding a full wave plate between the crossed polarizers results in beautifully vivid colors in the images as seen below.

Each of these images shows the result of a different combination of rotations of the full wave plate and the polarizer which sits atop the light source.  Note how both the background color and the colors in the crystal change.

 

Nature produces some crazy colors.

The image above is one of my cross polarization micro-photographs of some alum crystals on a microscope slide using a magnification of about 200x.  But what is causing those wild, saturated colors?  Some folks think the colors are either totally computer generated (CGI) or at least greatly exaggerated in post processing.  I assure you that they are neither, and if you looked at these crystals through a magnifying glass with your own eyes you would see the same colors, just as intense as in the images I post.

Over the next couple of posts I hope to give an idea of what is going on with out getting into a full blown physics dissertation on the subject.

Preparing A Sample

The process I use create these photos is straight forward and easy to do in practice.  First I dissolve some crystalline substance, like sugar or in this example alum, in water.  Place a few drops of the resulting solution onto a microscope slide, and as the water evaporates crystals will form.  I call this the sample which looks something like this.

 

Viewing With A Digital Microscope

The sample is then viewed with a digital microscope.  In ordinary light at a magnification of 30x the sample looks like this.

In ordinary light the sample is mostly transparent, though you can see some of the features which were produced as the crystals grew.  Interesting, but not particularly colorful.

Adding polarizers changes things…

Wikipedia has a good technical explanation of the physics of polarizers and cross polarization.  The upshot is that light has a property called polarization.  A linear polarizer is a material which blocks light whose polarization does not line up with the pass axis of the polarizer.  Polarized sunglasses reduce glare because it turns out that light which reflects off of shiny surfaces like water, or metals, ends up being polarized along the horizontal axis and the polarizer used in the lenses has been set to block horizontally polarized light.  Cross Polarization is a technique in which you use a pair of polarizers whose pass axes are set to be perpendicular to each other.  For example if one polarizer is set to block horizontally polarized light and the other is set to block vertically polarized light, no light would make it through the two polarizers.

Ordinarily putting your sample between a pair of crossed polarizers would result in a completely dark image.  However what we actually get looks like this.

Notice how everything around the crystal is dark, the crossed polarizers are indeed blocking light from passing through to the camera.  But why do we still see light which has passed through the alum crystal?  Not only is this light getting through, we are able to see many more details in the crystal including areas which are not colored in red and blue.

Birefringence

The reason we see this light is because the crystal is rotating the polarization of the light passing through it.  Even though the light entering the crystal may have been horizontally polarized after passing through the first linear polarizer, it can leave the crystal polarized along some other axis.  Some of this light will now be vertically polarized and able to pass through the second (crossed) polarizer.  How much the polarization of the light is rotated depends on several factors including both the thickness of the crystal and the color of the light.

This property of the crystal is called Birefringence, which also has a nice description in Wikipedia.  Not all materials are birefringent, most are not.  Crystalline materials however can be birefringent and the property is related to arrangement of atoms in the crystal itself.

Up to now I have described cross polarization for the case where the two polarizers are aligned perpendicular to each other, which completely blocks light which does not pass through the crystal.  However the two polarizers can be setup with any angle you want between their pass axes.  If they are setup with their pass axes parallel, all of the light passed by the first polarizer will also make it through the second.  If the angle between the two is something between 0º (parallel) and 90º (perpendicular) then a fraction of the light from the first polarizer will make it through the second one.  This changes the appearance of the crystal.  So by setting different rotations of the polarizers and the sample the light you see reveals different structures within the crystal as shown in the following images.

The use of cross polarization by itself can produce some beautiful images.  However the intensity of the colors seen can be greatly enhanced by adding one more optical element called a Full Wave Plate.  In my next post I show how this changes the images.