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In this German Wikipedia article about optical activity, I found the beautiful picture of starch granules:

enter image description here

Unfortunately, I have no idea what I am looking at. What are these crosses and why are they blue (i.e., same as the background color)?

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    $\begingroup$ You're looking at isogyres (so-called interference figures), which arise from light interference along different optical axes of your sample. They are commonly used to determine the crystal system of a sample when viewed using a petrographic microscope (or similar). See this Wikipedia page for relevant information. The blue color might arise from the use of a mica plate to examine the birefringence, or could be an artifact of the experiment (I'm a bit rushed now and haven't read you link in-depth). $\endgroup$ – Todd Minehardt Feb 3 '17 at 22:44
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    $\begingroup$ @ToddMinehardt That already seems like a pretty good answer to me, would be great if you found the time to write it up properly :) $\endgroup$ – tschoppi Apr 28 '17 at 17:29
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The figures you see are isogyres and arise in substances that are optically active, which means that polarized light is transmitted through the material in different ways depending on the orientation of the sample. These figures are called interference figures. I highly recommend that you peruse this excellent write-up on the optical properties of minerals, particularly the sections pertaining to uniaxial and biaxial crystals, birefringence, and how a petrographic microscope works.

From the Wikipedia entry on conoscopic interference pattern:

The figures are produced by optical interference when diverging light rays travel through an optically non-isotropic substance - that is, one in which the substance's refractive index varies in different directions within it.

Starch is uniaxial and the figures you see are characteristic of uniaxial materials when viewed using a petrographic microscope. See this reference (The Principles of Pharmacognocy. F. A. Flückiger and A. Tscirch. William Wood & Company, New York, 1887), pages 118-119.

As for the color seen in the image you reference: that arises from using a mica (or tint) plate which in turn indicates whether the substance is uniaxial positive or uniaxial negative, which refer to the propagation of light along differing optical axes. However, in your figure, the color of the isogyres and the background are both blue, which indicates to me that the blue color is simply a result of polarized light passing through the substance.

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