Radiography, also known as x-rays, is a technique to obtain images that are used to examine soft and bony tissues of the body. X-rays are short-wavelength electromagnetic waves produced when fast-moving electrons collide with substances in their pathways. X-rays travel in straight lines at the speed of light (186,000 miles/sec). When an x-ray beam passes through matter, some of its intensity is absorbed—the denser the matter, the greater the degree of x-ray absorption. The composite image produced represents these varying degrees of tissue density in shades of black, white, and gray. Images may be captured on photographic film, displayed on a video screen, or recorded on digital media. The basic principle of radiography is that differences in density among various body structures produce images of varying light or dark intensity, much like the negative print of a photograph. Dense structures appear white, whereas air-filled areas are black. Fluoroscopy is an imaging technique that uses x-rays to demonstrate movement of body organs and contrast agents.