Most people have seen a 2D X-ray, and have probably had one at some time in their life, either at a dentist’s office or in an emergency room. It is the kind of X-ray that looks like a picture, except that it shows hidden features that cannot be seen by the naked eye. In people, these are usually bones and dense tissue, but X-rays can also be used to detect defects in metal welds, bombs hidden in suitcases and clogs in water pipes.
X-rays are produced any time a high-energy electron strikes a metallic surface. The heart of an X-ray machine is the X-ray tube, which is a vacuum tube in which there is a cathode that excites and emits the electrons, and an anode, which attracts them and causes their energy to be converted into X-rays.
The cathode is heated to excite the electrons, and because it is negatively charged, it repels them into the empty space of the vacuum tube. The anode is a positively charged metallic electrode at the other end of the tube. Typically made of platinum, this electrode attracts the electrons. When an electron collides with the anode, an X-ray is emitted.
X-rays are capable of passing through objects that are opaque to visible light. Shorter wavelengths are more energetic than longer wavelengths, and X-rays have a wavelength in the range of a trillionth of a meter (0.000,000,000,1 m). The wavelength of visible light is about a thousand times greater, and its energy is correspondingly about one-thousandth as great.
To X-ray a human body, the energy of the X-rays must be great enough to easily pass through soft tissue, but not enough to pass through denser tissue and bone.
Because the human eye cannot see X-rays, they must be converted to visible light in order to be useful. The most common way of doing this is to use a photographic film that is sensitive to X-rays. It is also possible to use a glass screen coated with fluorescent substance. Such screens are called fluoroscopes. In the past, it was common to view fluoroscopes directly, but that put the person doing the viewing directly in the path of the X-rays, which could be very harmful with repeated exposure. Modern practice is to capture the image of the fluoroscope with a video camera and send it to a television screen where it can be safely viewed.