What are X rays? A simple introduction (2024)

What are X rays? A simple introduction (1)

by Chris Woodford. Last updated: October 31, 2022.

If our eyes could detect super-energetic forms of radiation such as X rays, looking at our friends would be an altogether more surreal experience: we'd beable to see straight through their skin and watch their bonesjiggling about underneath! Perhaps it's fortunate that we don't havethat kind of ability—but we can still enjoy the benefits of using Xrays all the same: they're hugely important in medicine, scientificresearch, astronomy, and industry. Let's take a closer look at what X rays are,how they work, and how we make them!

Photo: Once X rays had to be treated like old-fashioned photographs. Now, they're as easy to study and store as digital photographs on computer screens. Photo by Kasey Zickmund courtesy of U.S. Air Force.

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Contents

  1. What are X rays?
  2. What are X rays used for?
  3. How are X rays produced?
  4. How were X rays discovered?
  5. Find out more

What are X rays?

Imagine you had the job of redesigning light to make it a bit more powerful—so you couldsee through bodies, buildings, and anything else you fancied. You might come up with something a bit like X rays.

X rays are a kind of super-powerful version of ordinary light: a higher-energy form ofelectromagnetic radiation that travel at the speed of lightin straight lines (just like light waves do). If you could pinX rays down on a piece of paper and measure them, you'd find theirwavelength (the distance between one wave crest and the next)was thousands of times shorter than that of ordinary light. That means theirfrequency (how often they wiggle about) is correspondinglygreater. And, because the energy ofelectromagnetic waves is directly related to their frequency, X raysare much more energetic and penetrating than light waves as well.So here's the most important thing you need to remember: X rays can travelthrough things that ordinary light waves can't because they're much moreenergetic.

What are X rays? A simple introduction (2)

Artwork: The electromagnetic spectrum, with the X-ray band highlighted in yellow over toward the right.You can see that X rays have shorter wavelengths, higher frequencies, and higher energy than most othertypes of electromagnetic radiation, and don't penetrate Earth's atmosphere.Their wavelengths are around the same scale as atomic sizes.Artwork courtesy of NASA (please follow this link for a bigger and clearer version of this image).

We all know that some materials (such as glass and plastic) let light pass throughthem very easily while other materials (such as wood and metal)don't. In much the same way, there are materials that allow X rays topass straight through them—and materials that stop X rays dead intheir tracks. Why is this? When X rays enter a material, they have tofight their way through a huge scrum of atoms if they're going toemerge from the other side. What really gets in their way is theelectrons whizzing round those atoms. The more electrons there are,the more chance they have of absorbing the X rays and the less likely theX rays are to emerge from the material. X rays will tend to pass throughmaterials made from lighter atoms with relatively few electrons (suchas skin, built from carbon-based molecules), but they'restopped in their tracks by heavier atoms with lots of electrons.Lead, a heavy metal with 82 electrons spinning round each of itsatoms, is particularly good at stopping X rays. (That's why X-raytechnicians in hospitals wear lead aprons and stand behind leadscreens.) The fact that some materials let X rays travel through thembetter than others turns out to be very useful indeed.

What are X rays? A simple introduction (3)

Artwork: Lead is a heavy element that you'll find toward the bottomof the periodic table: its atoms contain lots of protons and neutrons, so they're very dense and heavy. Lead is very good at stopping X rays.

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What are X rays used for?

From studying tooth decay in your mouth to detecting events in distant galaxies, X rays are useful in many different ways.

Medicine

One of the first uses people found for X rays was in medicine—and they're still bestknown as a medical tool, used in both diagnosis and treatment. Hardmaterials such as bones and teeth are very good at absorbing X rays,whereas soft tissues like skin and muscle allow the rays to passstraight through. That makes X-ray photographs (which look likeshadows of the things inside your body) extremely useful for allkinds of medical diagnosis: they can show up broken bones, tumors, andalso help to diagnose lung conditions such as tuberculosisand pneumonia. Dental X rays help your dentist understand what's happening in parts of your mouth—inside your teeth and gums—that they could not otherwise see.

What are X rays? A simple introduction (4)

Photo: Taking a dental X ray with modern, digital technology.This equipment uses low-power (and therefore safer) X rays and instead of the dentist having to develop an old-fashioned photo, the results show up almost instantly on their computer screen. Photo by Matthew Lotz courtesy of US Air Force.

There's a limit to what a physician can understandfrom a two-dimensional photograph of your three-dimensional body,especially with so much packed inside such a small space, but3D-scanning technology helps to overcome that. CT or CAT(computerized axial tomography) scanners draw what areeffectively 3D, X-ray pictures on screens by firing pencil-thin beamsof X rays through a patient's body and using computer technology toturn lots of 2D pictures into a single 3D image.

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Photo: A typical CT scanner. The patient lies on the bed, which slidesthrough the hole in the donut-shaped scanner behind. The scanner unit contains one or more rotatingX-ray sources and detectors. Photo by Francisco V. Govea IIcourtesy of US Air Force and Wikimedia Commons.

Since X rays are highly energetic, they can damage living tissue when they passthrough it. On one hand, this means X rays have to be used cautiously and quiteselectively—and X-ray technicians (known as radiographers) have totake precautions about absorbing too much of the radiation duringtheir work. But on the other hand, X rays can also be used tosterilize medical equipment (because they destroy germs) and killtumors in the treatment of cancer. This is known asX-ray therapy (also called radiation therapy and radiotherapy).

Find out more about medical uses of X-rays from the US government'sNational Institute of Biomedical Imaging and Bioengineering (NIBIB).

Security

X ray scans that show up the organs lurking inside your body are just as useful forchecking bags at airport check-ins: X rays pass straight through softmaterials such as leather and plastic but are blocked by the metal inguns, knives, and weapons. Typically suitcases and bags travel upthrough large scanners on conveyor belts, with X ray images of theircontents appearing instantly on computer screens studied by securityguards. CT scans are increasingly being used in airport scanners tomeasure the density of liquids being carried in luggage; this hasproved to be a quick and effective way of detecting some kinds of explosives. Scanners such as this are called CTX machinesand are made by companies such as GE InVision.

What are X rays? A simple introduction (6)

Photo: Using digital X ray equipment (left) to check the contents of a suspicious package (on the floor, right). Photo by Jonathan Pomeroy courtesy of US Air Force.

Industrial applications

If you can use X rays to study lung problems or scan airport baggage, why not use itin a similar way to detect faults lurking inside machines? That's thetheory behind nondestructive testing, where engineers X ray all kinds of industrial equipment to help them track down things like cracksand fatigue in metal components that might otherwise go undetected.Turbine blades in airplane jet engines are tested in thisway to make sure they're not harboring any problems that would causethem to fail suddenly during flight. All kinds of other products arealso routinely studied with X rays. Oil paintings, for example, areoften X rayed to prove their authenticity (occasionally showing up earlier versions of a picture or entirely differentimages by the same artist on the same canvas).

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Photo: Nondestructive X ray testing is one way to inspect planes without taking them apart. Here, a plane has just been tested in a lead-lined hangar at Randolph US Air Force Base, Texas. The warning signs you can see on the door indicate the potential dangers from the X rays. Photo by Steve Thurow courtesy of US Air Force.

Tiny, precise X-ray beams can also be used as microscopic machine tools. Theminiature circuit patterns of integrated circuits (silicon chips) can now be drawnusing immensely precise beams of X rays using a technique called X-ray lithography. Light beamswere once used for this purpose; using X rays, which are thousands of times finer, allows components to be made smaller, which in turn makes for smaller and more powerful chips.

Scientific research

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Photo: Studying semiconductor materials with X-ray spectroscopy. Photo by Jim Yost courtesy ofUS DOE/NREL.

Apart from medicine, the other original use for X rays was in studying the innerstructure of materials. If you fire a beam of X rays at a crystal,the atoms scatter the beam in a very precise way,casting a kind of shadow of the crystal's interior pattern from whichyou can measure the distance between one atom and its neighbors. This iscalled X-ray diffraction orX-ray crystallography,and, thanks to British scientist Rosalind Franklin, it played a hugely important part in the discovery of DNA's structure in the 1950s.

Astronomy

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Photo: X-ray image of the Sun produced by the Soft X-ray Telescope (SXT).Photo courtesy of NASA Goddard Space Flight Center (NASA-GSFC).

We're used to the idea of looking through telescopes to see light from distantobjects—even ones far out into space. But not all telescopes workthis way. Radio telescopes, for example, are more like giantsatellite-dish antennas that capture radio waves being given off from those distant sources. X rays also travel through space and we can study them in a similar way with telescopes tuned torecognize their particular frequency. Unfortunately for astronomers,but possibly fortunately for the benefit of our own health, Earth'satmosphere absorbs X rays coming from space before they reach ourplanet's surface. That means we have to study sources of Xrays with telescopes located in space instead of ones based here on Earth.Find out more on NASA's page about X Ray Astronomy.

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How are X rays produced?

If you've read our main article on light, you'll understand that you see things whenthey reflect light rays. More specifically, reflection happens whenthe electrons in atoms inside objects move position to absorb and then re-emit lightenergy. If you want to make red light, you can shine a flashlight ona tomato so the red part of the original white light in yourflashlight beam is reflected back. X rays are produced in a moreenergetic version of the same process. If you want to make X rays,you simply fire a beam of really high-energy electrons (acceleratedusing a high-voltage electricity supply) at a piece ofmetal (typically tungsten). What gets reflected back, in this case,is neither light nor electrons but a beam of X rays. Generallyspeaking, the higher the voltage you use, the faster the electronsgo, the more energetically they crash into the tungsten, and thehigher the energy (and frequency) of the X rays they produce.

How were X rays discovered?

What are X rays? A simple introduction (10)

Photo: Wilhelm Röntgen's X-ray photograph of his wife's hand. Note the rings!Photo believed to be in the public domain, courtesy of the National Library of Medicine's Images from the History of Medicine (NLM) collection and the National Institutes of Health.

Here's a brief history of X rays from their discovery, at the end of the 19thcentury, up to modern times:

19th century

  • 1895: German physicist Wilhelm Röntgen (1845–1923) discovers X rays while experimentingwith cathode rays (the name then given to electron beams) in a glasstube. The X rays leak through the glass and into a nearby cardboardbox, where they make paper coated with a fluorescent material glow. Röntgen doesn'tknow what these rays are so he calls them "X rays" (X being thename typically given to unknown quantities in mathematicalproblems). This discovery earns him the very firstNobel Prize in Physics in 1901.
  • 1896: Inspired by this discovery, prolific American inventor Thomas Edison (1847–1931) develops an X-ray viewercalled a fluoroscope.

20th century

  • 1906: Charles Barkla(1877–1944), a British physicist, shows that X rays can bepolarized in a similar way to beams of light. This providesimportant evidence that X rays are essentially like light waves onlyof different wavelength and frequency.
  • 1912: German physicist Max von Laue (1879–1960) discovers he can measure the wavelength of X rays by firing them through crystals, roughly confirming thewavelength of X rays and the regular atomic nature of crystals.
  • 1913-1914: British physicist William Henry Bragg (1862–1942) and his son(William) Lawrence Bragg (1890–1971)effectively reverse this experiment, showing how X rays of known wavelength can be used tomeasure the atomic spacing of crystals—and developing the field of X-ray crystallography. For this,they earn the 1915 Nobel Prize in Physics.
  • 1913: American physicist William David Coolidge (1873–1975) develops the practical X-ray-making machine. Known as a Coolidge tube,it's a long glass jar with an electron beam and a metal targetinside. When the beam is fired at the target, X rays are produced.Increasing the voltage produces faster and more energetic X rayswith higher frequencies and shorter wavelengths. Coolidge patentshis invention in 1916. Most X-ray machines still work broadly this way today.

    What are X rays? A simple introduction (11)

    Illustration: A typical Coolidge tube. Artwork courtesy of theWellcome Collection published under a Creative Commons (CC BY 4.0) licence.

  • 1922: Arthur H. Compton (1892–1962), another American physicist, studies the reflection of X rays from highly polished glass and measures their wavelengthvery precisely. He discovers the phenomenon now called the Compton effect (or Compton scattering): the scattered X rays have less energy than the particles in the original beam, providing evidence for the particle-nature of electromagnetic radiation.
  • 1953: Francis Crick (1916–2004) and James D. Watson (1928–)work out the structure of DNA with help from X-ray diffraction images produced by Rosalind Franklin (1920–1958).
  • 1972: British electronics engineer Godfrey Hounsfield (1919–2004) invents the CT scanner, which makes 3D images of the inside of aperson's body using thin X-ray beams.
  • 1980s: Powerful X-ray lasers are proposed that would produce X rays through a process ofstimulated emission (where atoms are made to emit radiation in a precise way by persistently "pumping" them with energy in aspace between two parallel mirrors).
  • 1999: The Space Shuttle launches the Chandra X-ray Observatory—the most sensitive X-ray telescope to date.

    What are X rays? A simple introduction (12)

    Photo: The Chandra X-ray telescope just before it was released from the Space Shuttle Columbia on on July 23, 1999. Photo courtesy of NASA/JSC

21st century

  • 2000s: CT X-ray scanners are used to improve baggage-screening security in airports.
  • 2009: Scientists at SLAC National Accelerator Laboratory, Menlo Park, California produce a powerful X-ray laser described as"the world's brightest X-ray source."
  • 2018: Researchers in New Zealand develop a medical scanner that can produce 3D color X rays of the human body.
  • 2019: Singapore scientists demonstrate how perovskite crystals could make better X ray detectors.

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On this website

  • Atoms
  • Electromagnetic spectrum
  • Light
  • Magnetism
  • Space telescopes

On other sites

  • X Rays: What does an X ray test involve? What will it feel like? Are there any risks? The U.S. National Library of Medicine's Medline tells you all you need to know.
  • X Rays: The UK government's NHS website sets out the procedure of having an X ray and describes the benefits and risks (compared to other natural risks we all experience every day).

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