 Light is a form of electromagnetic radiation. At some wavelengths, it appears as visible light. At others, it takes the form of radiation we cannot see, such as ultraviolet, infrared, and x-rays. X-rays have very short wavelengths that, unlike visible light, are able to pass right through most tissues in the human body. As bones contain calcium, a denser material than most other tissues, they are able to stop some of the x-rays and can thus form a shadow. If you place your hand in front of a flashlight, it will cast a shadow on the wall behind it. In a similar manner, x-rays directed at the body produce shadows on the opposite side that can be registered on film or on a digital sensor, so that bones are clearly visible while soft tissues, such as muscle, are shadowy. As a result, the two-dimensional images formed by the x-rays are quite useful in revealing structures within the body and can easily display fractures in bones.