by Chris Woodford. Last updated: June 25, 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 be
able to see straight through their skin and watch their bones
jiggling about underneath! Perhaps it's fortunate that we don't have
that kind of ability—but we can still enjoy the benefits of using X
rays all the same: they're hugely important in medicine, scientific
research, 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.
What are X rays?
Imagine you had the job of redesigning light to make it a bit more powerful—so you could
see 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 of
electromagnetic radiation that travel at the speed of light
in straight lines (just like light waves do). If you could pin
X rays down on a piece of paper and measure them, you'd find their
wavelength (the distance between one wave crest and the next)
was thousands of times shorter than that of ordinary light. That means their
frequency (how often they wiggle about) is correspondingly
greater. And, because the energy of
electromagnetic waves is directly related to their frequency, X rays
are much more energetic and penetrating than light waves as well.
So here's the most important thing you need to remember: X rays can travel
through things that ordinary light waves can't because they're much more
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 other
types 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 through
them very easily while other materials (such as wood and metal)
don't. In much the same way, there are materials that allow X rays to
pass straight through them—and materials that stop X rays dead in
their tracks. Why is this? When X rays enter a material, they have to
fight their way through a huge scrum of atoms if they're going to
emerge from the other side. What really gets in their way is the
electrons whizzing round those atoms. The more electrons there are,
the more chance they have of absorbing the X rays and the less likely the
X rays are to emerge from the material. X rays will tend to pass through
materials made from lighter atoms with relatively few electrons (such
as skin, built from carbon-based molecules), but they're
stopped in their tracks by heavier atoms with lots of electrons.
Lead, a heavy metal with 82 electrons spinning round each of its
atoms, is particularly good at stopping X rays. (That's why X-ray
technicians in hospitals wear lead aprons and stand behind lead
screens.) The fact that some materials let X rays travel through them
better than others turns out to be very useful indeed.
Artwork: Lead is a heavy element that you'll find toward the bottom
of 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.
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.
One of the first uses people found for X rays was in medicine—and they're still best
known as a medical tool, used in both diagnosis and treatment. Hard
materials such as bones and teeth are very good at absorbing X rays,
whereas soft tissues like skin and muscle allow the rays to pass
straight through. That makes X-ray photographs (which look like
shadows of the things inside your body) extremely useful for all
kinds of medical diagnosis: they can show up
also help to diagnose lung conditions such as
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.
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 understand
from a two-dimensional photograph of your three-dimensional body,
especially with so much packed inside such a small space, but
3D-scanning technology helps to overcome that. CT or CAT
(computerized axial tomography) scanners draw what are
effectively 3D, X-ray pictures on screens by firing pencil-thin beams
of X rays through a patient's body and using computer technology to
turn lots of 2D pictures into a single 3D image.
Photo: A typical chest X ray. This one (apparently) shows
emphysema bubbles in the patient's left lung, which are severely affecting breathing.
Photo courtesy of National Heart, Lung and Blood Institute (NHLBI) and
National Institutes of Health.
Since X rays are highly energetic, they can damage living tissue when they pass
through it. On one hand, this means X rays have to be used cautiously and quite
selectively—and X-ray technicians (known as radiographers) have to
take precautions about absorbing too much of the radiation during
their work. But on the other hand, X rays can also be used to
sterilize medical equipment (because they destroy germs) and kill
tumors in the treatment of cancer. This is known as
X-ray therapy (also called radiation therapy and radiotherapy).
Find out more about medical uses of X-rays from the US government's
National Institute of Biomedical Imaging and Bioengineering (NIBIB).
X ray scans that show up the organs lurking inside your body are just as useful for
checking bags at airport check-ins: X rays pass straight through soft
materials such as leather and plastic but are blocked by the metal in
guns, knives, and weapons. Typically suitcases and bags travel up
through large scanners on conveyor belts, with X ray images of their
contents appearing instantly on computer screens studied by security
guards. CT scans are increasingly being used in airport scanners to
measure the density of liquids being carried in luggage; this has
proved to be a quick and effective way of detecting some kinds of explosives. Scanners such as this are called CTX machines
and are made by companies such as GE InVision.
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.
If you can use X rays to study lung problems or scan airport baggage, why not use it
in a similar way to detect faults lurking inside machines? That's the
nondestructive testing, where engineers X ray all kinds of industrial equipment to help them track down things like cracks
and fatigue in metal components that might otherwise go undetected.
Turbine blades in airplane jet engines are tested in this
way to make sure they're not harboring any problems that would cause
them to fail suddenly during flight. All kinds of other products are
also routinely studied with X rays. Oil paintings, for example, are
often X rayed to prove their authenticity (occasionally showing up earlier versions of a picture or entirely different
images by the same artist on the same canvas).
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. The
miniature circuit patterns of integrated circuits (silicon chips) can now be drawn
using immensely precise beams of X rays using a technique called X-ray lithography. Light beams
were 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.
Photo: Studying semiconductor materials with X-ray spectroscopy. Photo by Jim Yost courtesy of
Apart from medicine, the other original use for X rays was in studying the inner
structure 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 which
you can measure the distance between one atom and its neighbors. This is
called X-ray diffraction or
and, thanks to British scientist
Franklin, it played a hugely important part in the discovery of DNA's structure in the 1950s.
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 distant
objects—even ones far out into space. But not all telescopes work
this way. Radio telescopes, for example, are more like giant
satellite-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 to
recognize their particular frequency. Unfortunately for astronomers,
but possibly fortunately for the benefit of our own health, Earth's
atmosphere absorbs X rays coming from space before they reach our
planet's surface. That means we have to study sources of X
rays with telescopes located in space instead of ones based here on Earth.
Find out more on NASA's page about X Ray Astronomy.
How are X rays produced?
If you've read our main article on light, you'll understand that you see things when
they reflect light rays. More specifically, reflection happens when
the electrons in atoms inside objects move position to absorb and then re-emit light
energy. If you want to make red light, you can shine a flashlight on
a tomato so the red part of the original white light in your
flashlight beam is reflected back. X rays are produced in a more
energetic version of the same process. If you want to make X rays,
you simply fire a beam of really high-energy electrons (accelerated
using a high-voltage electricity supply) at a piece of
metal (typically tungsten). What gets reflected back, in this case,
is neither light nor electrons but a beam of X rays. Generally
speaking, the higher the voltage you use, the faster the electrons
go, the more energetically they crash into the tungsten, and the
higher the energy (and frequency) of the X rays they produce.
How were X rays discovered?
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 19th
century, up to modern times:
- 1895: German physicist Wilhelm Röntgen (1845–1923) discovers X rays while experimenting
with cathode rays (the name then given to electron beams) in a glass
tube. The X rays leak through the glass and into a nearby cardboard
box, where they make paper coated with a fluorescent material glow. Röntgen doesn't
know what these rays are so he calls them "X rays" (X being the
name typically given to unknown quantities in mathematical
problems). This discovery earns him the very first
Nobel Prize in Physics in 1901.
- 1896: Inspired by this discovery, prolific American inventor Thomas Edison (1847–1931) develops an X-ray viewer
called a fluoroscope.
- 1906: Charles Barkla
(1877–1944), a British physicist, shows that X rays can be
polarized in a similar way to beams of light. This provides
important evidence that X rays are essentially like light waves only
of 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 the
wavelength 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 to
measure 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 target
inside. When the beam is fired at the target, X rays are produced.
Increasing the voltage produces faster and more energetic X rays
with higher frequencies and shorter wavelengths. Coolidge patents
his invention in 1916. Most X-ray machines still work broadly this way today.
Illustration: A typical Coolidge tube. Artwork courtesy of the
Wellcome 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 wavelength
very 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 a
person's body using thin X-ray beams.
- 1980s: Powerful X-ray lasers are proposed that would produce X rays through a process of
stimulated emission (where atoms are made to emit radiation in a precise way by persistently "pumping" them with energy in a
space between two parallel mirrors).
- 1999: The Space Shuttle launches the
Chandra X-ray Observatory—the most sensitive X-ray telescope to date.
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
- 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.