X Ray Definition and Properties (X Radiation)

What You Need to Know About X-Rays

Dense tissue scatters x-rays, producing an image, while the radiation passes through soft tissue.
Dense tissue scatters x-rays, producing an image, while the radiation passes through soft tissue. Tetra Images / Getty Images

X-rays or x-radiation are part of the electromagnetic spectrum with shorter wavelengths (higher frequency) than visible light. X-radiation wavelength ranges from 0.01 to 10 nanometers, or frequencies from 3×1016 Hz to 3×1019 Hz. This puts the x-ray wavelength between ultraviolet light and gamma rays. The distinction between x-ray and gamma rays may be based on wavelength or on radiation source. Sometimes x-radiation is considered to be radiation emitted by electrons, while gamma radiation is emitted by the atomic nucleus.

German scientist Wilhelm Röntgen was the first to study x-rays (1895), although he was not the first person to observe them. X-rays had been observed emanating from Crookes tubes, which were invented circa 1875. Röntgen called the light "X-radiation" to indicate it was a previously unknown type. Sometimes the radiation is called Röntgen or Roentgen radiation, after the scientist. Accepted spellings include x rays, x-rays, xrays, and X rays (and radiation).

The term x-ray is also used to refer to a radiographic image formed using x-radiation and to the method used to produce the image.

Hard and Soft X-Rays

X-rays range in energy from 100 eV to 100 keV (below 0.2–0.1 nm wavelength). Hard x-rays are those with photon energies greater than 5-10 keV.  Soft x-rays are those with lower energy. The wavelength of hard x-rays is comparable to the diameter of an atom. Hard x-rays have sufficient energy to penetrate matter, while soft x-rays are absorbed in air or penetrate water do a depth of about 1 micrometer.

Sources of X-Rays

X-rays may be emitted whenever sufficiently energetic charged particles strike matter. Accelerated electrons are used to produce x-radiation in an x-ray tube, which is a vacuum tube with a hot cathode and a metal target. Protons or other positive ions may also be used. For example, proton-induced x-ray emission is an analytical technique.

Natural sources of x-radiation include radon gas, other radioisotopes, lightning, and cosmic rays.

How X-Radiation Interacts With Matter

The three ways x-rays interact with matter are Compton scattering, Rayleigh scattering, and photoabsorption. Compton scattering is the primary interaction involving high energy hard x-rays, while photoabsorption is the dominant interaction with soft x-rays and lower energy hard x-rays. Any x-ray has sufficient energy to overcome the binding energy between atoms in molecules, so the effect depends on the elemental composition of matter and not its chemical properties.

Uses of X-Rays

Most people are familiar with x-rays because of their use in medical imaging, but there are many other applications of the radiation:

In diagnostic medicine, x-rays are used to view bone structures. Hard x-radiation is used to minimize absorption of low energy x-rays. A filter is placed over the x-ray tube to prevent transmission of the lower energy radiation. The high atomic mass of calcium atoms in teeth and bones absorbs x-radiation, allowing most of the other radiation to pass through the body. Computer tomography (CT scans), fluoroscopy, and radiotherapy are other x-radiation diagnostic techniques.

X-rays may also be used for therapeutic techniques, such as cancer treatments.

X-rays are used for crystallography, astronomy, microscopy, industrial radiography, airport security, spectroscopy, fluorescence, and to implode fission devices. X-rays may be used to create art and also to analyze paintings. Banned uses include x-ray hair removal and shoe-fitting fluoroscopes, which were both popular in the 1920s.

Risks Associated with X-Radiation

X-rays are a form of ionizing radiation, able to break chemical bonds and ionize atoms. When x-rays were first discovered, people suffered radiation burns and hair loss. There were even reports of deaths. While radiation sickness is largely a thing of the past, medical x-rays are significant source of man-made radiation exposure, accounting for about half the total radiation exposure from all sources in the U.S. in 2006.

There is disagreement about the dose that presents a hazard, partially because risk depends on multiple factors. It is clear x-radiation is capable of causing genetic damage that can lead to cancer and developmental problems. The highest risk is to a fetus or child.

Seeing X-Rays

While x-rays are outside the visible spectrum, it's possible to see the glow of ionized air molecules around an intense x-ray beam. It's also possible to "see" x-rays if a strong source is viewed by a dark-adapted eye. The mechanism for this phenomenon remains unexplained (and the experiment is too dangerous to perform). Early researchers reported seeing a blue-gray glow that seemed to come from within the eye.


Medical Radiation Exposure of the U.S. Population Greatly Increased Since the Early 1980s, Science Daily, March 5, 2009. Retrieved July 4, 2017.