Doctors have used x-rays for over a century to see inside the body. X-rays can diagnose a variety of problems, including bone fractures, arthritis, cancer, and pneumonia. During this test, you usually stand in front of a photographic plate while a machine sends x-rays, a type of radiation, through a part of your body. Originally, a photograph of internal structures was produced on film; nowadays, the image created by the x-rays goes directly into a computer.
Dense structures absorb many of the x-rays and block them from reaching the plate. Calcium in bones is dense, so it absorbs lots of x-rays, making the image of the bone appear white. Fat and other soft tissues are less dense, so they allow more radiation to pass through them and appear in shades of gray. Hollow body parts, such as the lungs, appear dark or black because lots of x-rays pass through them. (In some other countries, like the United Kingdom, the colors are reversed, and dense structures are black.)