What are X-rays?
An X-ray picture is really a picture of the shadows cast by the denser materials (like bones) in your body.

These shadows are projected onto a film that has been coated with a sensitive material. The film is developed in a manner very similar to a photograph.

If the doctor suspects that you have a broken bone he will probably take an X-ray picture to check. X-rays are made of the same electromagnetic particles as light but with a much shorter wavelength. These shorter wavelengths allow X-rays to pass through the human body.

X-rays are produced in a special tube. Most of the air is pumped out of the tube. The tube holds a negatively charged electrode called a cathode. Inside the cathode is a tungsten wire which will give off electrons when heated. The tube also contains an anode, or target electrode. The anode is also made of tungsten. When the electrons emitted by the cathode hit the tungsten anode they are stopped abruptly and some of their energy is turned into X-radiation.

X-rays were named by their discoverer, Wilhelm Roentgen. He used the mathematical symbol X, which stands for an unknown, to denote his unknown rays.

History
X-rays were first discovered in 1895 when, during experiments with electric currents passed through a vacuum tube, Wilhelm Conrad Röntgen noted that a nearby fluorescent screen glowed when the current was being passed. When the current was switched off the screen stopped glowing. He attributed this effect to previously unknown rays which, X being the symbol for an unknown quantity, he called X-rays.

We now know that these rays are, like light and radio waves, a form of electromagnetic radiation. X-rays have high energy and short wavelength and are able to pass through tissue. On their passage through the body, the denser tissues, such as the bones, will block more of the rays than will the less dense tissues, such as the lung.

A special type of photographic film is used to record X-ray pictures. The X-rays are converted into light and the more energy that has reached the recording system, the darker that region of the film will be. This is why the bones on an X-ray image appear whiter (less energy passes through) than the lungs (more energy passes through).

In the early days of X-rays, images, such as that of Mrs Röntgen's hand, were produced differently: the bones appeared dark rather than white.

Within two months of their discovery, X-rays were being used both in Europe and North America, not just to take pictures of the internal organs of living people but also to treat a wide variety of diseases. The energy that does not pass through the body is deposited within it and it is this energy that causes the biological effects of radiation.

The machines used to take X-ray pictures produce X-rays with energies of around 120,000 electron volts. The X-rays used for cancer treatment are much more powerful, with energies of between 2 million and 20 million electron volts.
 

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