This module is designed to introduce students to two major tools that are used to determine the structure of matter and to reinforce concepts associated with electromagnetic radiation. Historically, X-ray and related diffraction methods have provided a great deal of information about crystalline materials ranging from gold to table salt to DNA, in which the atoms or molecules are arranged in a repeating pattern. Because the wavelength of X-rays and the spacing between atoms in a crystal are comparable, the phenomenon of diffraction occurs. From the pattern of diffraction spots, scientists work backward to identify what atoms are present in the crystal and how they are arranged relative to one another. This experiment is expensive and dangerous to demonstrate in a classroom, as X-rays are a highly energetic form of electromagnetic radiation.