A crystal is a solid where the individual molecules are arranged in a highly ordered three-dimensional structure like we see in grains of sugar and table salt. This is compared to when a substance is dissolved in a liquid and all the individual molecules are floating randomly around. If a liquid with a crystallizable substance is left to evaporate. The individual molecules will rearrange themselves and get bound to one another in a crystalline structure. Depending on which substance it is, both the micro- and macroscopic crystal structure will vary greatly.
Some crystals are what is known as birefringent. This means that light waves will behave differently depending on the way they travel through the crystal. However, this is not noticed in everyday life because most light is a mix of light waves with different orientations. But polarized light has been filtered to only allow light waves with a specific orientation to pass through. So, when a birefringent crystal is illuminated with a polarized light, the individual light waves interact differently with the crystals altering the waves a bit in respect to each other and depending on the thickness and orientation of the crystals we get a change in color.
In microscopy, we sometimes use this interaction of polarized light and birefringent crystals for microphotography. This is able to produce a variety of different beautiful patterns and colors depending on which substance is crystalized. One of the most popular is vitamin C, also known as ascorbic acid. This compound is both easy to crystalize, non-toxic, and easily available in its pure form which is great for microscopists, both amateur and professional. Another, more tricky combination of compounds is the mix of the amino acids l-glutamine and beta-alanine. These are use as food supplements and are also non-toxic, but to get them to crystalize properly can be a bit of a challenge because a lot of things will interact and affect the crystallization process. Like the solvent, speed of evaporation, purity of the substances, temperature and humidity etc.
In the clip you are able to see time lapse footage of different compounds crystallizing.
To polarize the light coming from the microscope light source a polarizer is used which is placed in the light path, before the specimen. When the light then travels through and interact with the specimen the light waves will get changed, and to see the changes we need to filter out all “unwanted” changes. This is done with an analyzer which is a second polarizing filter placed in the light path after the specimen. Motic carries a broad selection of both polarizers and analyzers which can be found here.
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