A doctor's x-ray machine consists of two parts: an x-ray source at one end, and a camera at the other. The arm or mouth or other body part to be examined is placed in between these two parts. X-rays from the source shine through the impeding body part, and the camera records the x-rays that reach the photographic film inside. Bone is denser than muscle tissue and skin, so it stops more of the x-rays (and hence fewer x-rays make it to the region of the film that's behind the bone). In the developed film negative, the bone looks lighter than the rest of the picture, since that part of the film was exposed to fewer x-rays.
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