X-rays are a form of radiation, like light or radio waves, which can be focused into a beam, much like a flashlight beam. Unlike a beam of light, however, X-rays can pass through most objects, including the human body. When X-rays strike a piece of photographic film, they can produce a picture. Dense tissues in the body, such as bones, block (absorb) many of the X-rays and appear white on an X-ray picture. Less dense tissues, such as muscles and organs, block fewer of the X-rays (more of the X-rays pass through) and appear in shades of gray. X-rays that pass only through air appear black.