In 1895, physicist Wilhelm Roentgen was intrigued by glowing cathode tubes and decided to see what they could do. He found that the rays they emitted could pass through certain solid objects and leave a shadowy image of that object on a fluorescent screen. He was even more amazed to find that when the rays passed through body parts, such as his hand, the bones beneath the skin became clearly visible on the screen. Because he didn't know exactly what was causing this phenomenon, he labeled the rays " X," which is the mathematical symbol for anything that is unknown.
Scientists today know that X-rays are a form of energy that travels in waves. X-rays can enter solid objects, where they either are absorbed or continue to pass through the object. The denser the material X-rays enter, the more they are absorbed and the less they are able to pass through.
Teeth and bone are very dense, so they absorb X-rays, but gums and cheeks are much less dense, so X-rays pass through more easily. That's why cheeks and gums appear dark and without detail on the X-ray film, but teeth show up much lighter. And fillings, which are even denser than bone, will show up as a solid, bright white area. Dental caries (cavities) will show up on an X-ray as a darker patch in a light tooth.
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