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Tag: x-ray

  • A History of Crystallography – Part Three

    Last week, we discussed crystalline structure classifications and the discovery of X-rays and their use in crystallography.

    Bragg’s Law and X-ray Diffraction

    Imagine you have a laser pointer in a room of spherical mirrors and black walls. Depending on where you point the laser pointer, it will hit the wall in a different position. Knowing the angle you shine the laser pointer and where it lands, you can find the distance between two of the mirrors. Now, replace the mirrors with atoms and the laser with an X-ray, and you have a setup to find the distance between each atom. The formula used to find that distance is called Bragg’s law.

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  • A History of Crystallography – Part Two

    Last week, we talked about Kepler’s observations on the lattice structure of snowflakes, Steno’s law, and Rene Just Huay’s fundamental unit hypothesis.

    Crystal Classification

    In 1830, a few decades after Huay’s fundamental unit hypothesis, Johann F. C. Hessel used geometry to derive the possible unit structures of crystals. He knew that crystals could only have certain types of rotation, two-fold, three-fold, four-fold, and six-fold. That means, starting from the center of the unit, drawing lines that split the crystal into two, three, four, or six equal sections. Think of it like a circle; starting from the center, the number of lines to the edge of the circle you draw while making sectors of the same shape is the number of rotational symmetry. Because lattices have a limited number of rotational axes, even without a microscope, Hessel was able to describe all possible fundamental unit symmetries. Auguste Bravais described many of these symmetries more specifically as types of lattice structures.

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