Final answer:
Items that cannot pass through an X-ray machine are generally known as Prohibited Items. X-rays form images based on material density, and are widely used in airport security, as well as medical and industrial fields.
Step-by-step explanation:
Items that cannot go through an X-ray machine are typically classified as Prohibited Items. This may include materials that are dangerous when exposed to X-rays, as well as items that may be damaged by X-ray radiation. The nature of X-ray technology allows it to create images by the shadows cast by different materials.
More specifically, X-rays penetrate materials and the darker shadows on an X-ray image correspond to denser materials. Thus, X-ray images are extremely useful in security contexts, like examining luggage at airports, highlighted in Figure 8.26 and Figure 30.26, as well as medical diagnostics and industrial applications such as detecting cracks in critical components.