An X-ray machine is essentially a camera. Instead of visible light, however, it uses X-rays to expose the film.

X-rays are like light in that they are electromagnetic waves, but they are more energetic so they can penetrate many materials to varying degrees. When the X-rays hit the film, they expose it just as light would. Since bone, fat, muscle, tumors and other masses all absorb X-rays at different levels, the image on the film lets you see different (distinct) structures inside the body because of the different levels of exposure on the film.

As with many of mankind's monumental discoveries, X-ray technology was invented completely by accident. In 1895, a German physicist named Wilhelm Roentgen made the discovery while experimenting with electron beams in a gas discharge tube. Roentgen noticed that a fluorescent screen in his lab started to glow when the electron beam was turned on. This response in itself wasn't so surprising -- fluorescent material normally glows in reaction to electromagnetic radiation -- but Roentgen's tube was surrounded by heavy black cardboard. Roentgen assumed this would have blocked most of the radiation.

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