X-rays are a form of electromagnetic radiation, just like visible light, but with much shorter wavelengths. This means that they can pass through many objects that visible light cannot, including clothing, skin, and most other soft materials. However, x-rays are absorbed by dense materials, such as metal, bone, and certain chemicals.

The x-ray machine at an airport uses this property of x-rays to create images of the contents of your luggage. The x-rays pass through your luggage and are absorbed by dense objects, creating a shadow on the detector. This shadow can be used to identify the shape and size of the objects in your luggage.