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by Ron Kurtus (revised 15 February 2016)

X-rays consist of high energy radiation. They are part of the electromagnetic spectrum, which also includes radio waves, microwaves, and visible light.

X-rays have an extremely short wavelength or high frequency, thus giving them high energy. X-rays can penetrate most materials except lead shielding. When they were discovered, the "X" stood for "unknown," because they were so mysterious. The name has been used ever since.

X-rays are usually generated by colliding high energy electrons against a metal target in an evacuated tube. Detection is usually done with a photographic film or detector. X-rays are used in medicine and industry to examine structural problems.

(Note: For information on how much radiation is safe, see X-ray Health Risks)

Questions you may have include :

This lesson will answer those questions. Useful tool: Units Conversion

Discovery of x-rays

X-rays were discovered accidentally by Wilhelm Roentgen in 1895 when he was experimenting with a gas-discharge tube (something like a neon light). He noticed some fluorescent paper across the room start to glow, apparently from some unknown radiation coming from the tube. He called it X-radiation. The "X" stood for unknown, because the radiation was so mysterious. The name has been used ever since.

Soon it was found that x-rays would pass through the soft tissues of the body and make images on a photographic film. In the early 1900s, legislators tried to outlaw x-rays because they were afraid scientists would be able to use them to see through people's clothes. They did not understand that although x-rays would go through clothes, the only thing that could be seen would be the person's skeleton.

Creating and detecting x-rays

X-rays are typically created or generated by aiming a high energy electron beam at a metal target. X-rays are given off as a result of the collision. This is usually done in an evacuated tube. Common methods of generating x-rays are called the Bremsstrahlung process, K-shell emission and the synchrotron effect.

(See X-ray Generation for more information.)

Television tube

Note that the way your television tube works is that electrons are shot to the screen, where a fluorescence material glows when it is hit by the electrons, creating the image on the TV screen. These electrons have high enough in energy to create low level x-rays at the screen.

In early televisions, it was not advisable to sit real close to the TV screens. Today, the x-rays are only measurable a few inches from the screen.

Sun gives off x-rays

Since the Sun has extremely high energy electrons, it is a source of natural x-rays. Their energy is greatly diminished by the time they get to the Earth.

Detection with photographic film

A common way to detect x-rays is with photographic film. Since x-rays are electromagnetic waves just like visible light rays, they also will cause photographic film to be exposed. Usually, a special film that is more sensitive to the x-ray wavelengths is used.

People used to have problems of having their film exposed when putting a camera through an x-ray machine at airport luggage checks. Now airlines claim the x-ray intensity is so low that it will not affect camera film. But you still should use caution.

A new way to detect x-rays that some doctors and dentists are using is with a digital detector, similar to one used in your digital camera. This allows them to see the image immediately, instead of having to wait to have the film developed. Unfortunately, the x-ray detectors are quite expensive, so many are not using them yet. (My dentist said his detector cost $9000. And that is why he has to charge me so much!)

Uses of x-rays

There are a variety of uses of x-rays.

Examine luggage and cargo

X-rays are being used in airports to examine luggage for weapons or bombs. Note that the metal detector that you walk through in the airport does not x-ray you. It uses magnetic waves to detect metal objects.

X-rays are also being used to examine cargo luggage for illegal or dangerous material.

Used in industry

Another use of x-rays is in industry. They can be used to detect structural problems and cracks in metals that cannot be seen from the outside. X-rays are used on commercial airplanes and bridges to make sure there are no stress fractures or other dangerous cracks in the material.

Medical use

The most common use of x-rays is in medicine and dentistry. X-rays are used to examine inside the body to try to see if there is anything abnormal. Broken bones, cancerous growths, and tooth decay are some of the problems that can be detected by an x-ray of a person.

Excessive use can be dangerous

Since x-rays have a very short wavelength, they pack a lot more energy that radiation with longer wavelengths. Although x-rays pass through the body, they also can cause harm by altering atoms or molecules they happen to hit.

If a person is exposed to high intensity x-rays often or over a long period of time, there is the potential of the person developing cancer in the exposed area.

Medical x-rays safe

Medical and dental x-rays are very low intensity, so the hazard is minimal. Still, x-ray technicians go behind a lead shield when giving x-rays because of the frequency of exposure. A person can receive many medical or dental x-rays in a year with very little risk of getting cancer from it. In fact, exposure to natural radiation—such as cosmic rays from space—pose a greater risk.


X-rays are electromagnetic waves with extremely short wavelengths. They can pass though many materials, but are stopped by lead. When they were discovered, the "X" stood for "unknown," because they were so mysterious. X-rays are usually generated by aiming high energy electrons at a metal target. Detection is usually done with a photographic film.

X-rays are used in industry to examine metal for cracks and stress. They are also used extensively in medicine and dentistry to examine for broken bones and disease. Excessive exposure to x-rays can harm a person's health, but most medical practitioners are careful not to exceed suggested limits.

Look through your problems with x-ray vision

Resources and references

Ron Kurtus' Credentials


Electromagnetic Waves Resources

Physics Resources


Top-rated books on X-Rays

Top-rated books on Electromagnetic Waves

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