X-rays, Light and Electromagnetic Waves

X-rays

X-rays are part of the electromagnetic spectrum, which includes radio waves, microwaves, and visible light. They have an extremely short wavelength or high frequency, thus giving them high energy. X-rays can penetrate most materials except lead shielding. They are used in medicine and industry to examine structural problems. When they were discovered, the "X" stood for "unknown," because they were so mysterious. The name has been used ever since.

Questions you may have include :

How are x-rays created and detected?
What are their physical characteristics?
How are x-rays used?

Creating and detecting x-rays

When a high energy electron is accelerated into a metal target, x-rays are given off as a result of the collision. This is usually done in an evacuated tube. It is caused when the high energy electron knocks the electron from an inner orbit in an atom. When the atom replaces that electron, it gives off the x-ray.

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.

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.

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.

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 I would still 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!)

Characteristics of x-rays

X-rays are part of the electromagnetic spectrum, which includes radio, microwaves, visible light, and ultraviolet rays. They have the same general characteristics as other waveforms, but x-rays also have specific characteristics related to their wavelength.

Like every waveform, x-rays have velocity, wavelength, frequency and amplitude.

Since x-rays are electromagnetic radiation, their velocity in a vacuum is the same as visible light: 186,000 miles/second or 300,000 kilometers/sec. Its velocity through transparent matter is less, according to the index of refraction for that material at that wavelength.

X-rays have a very short wavelength compared to other electromagnetic waves.

**Wavelength Comparison**
Waveform	Average Wavelength
Radio	Approx. 1.5 kilometer or 1 mile = 1.5 x 10⁵cm
Visible Light	1/1000 centimeter = 10^-3cm
X-rays	1/1000000 centimeter = 10^-6cm

Only gamma rays coming from atomic explosions have a shorter wavelength than x-rays.

The frequency of x-rays is its velocity divided by wavelength:

Frequency = Velocity / Wavelength

The amplitude of an x-ray is equivalent to its intensity or brightness. Just like a bright visible sunlight can cause a burn, so too can an intense x-ray burn the skin.

The way the various types of electromagnetic radiation interact with matter is determined by their wavelength. Since x-rays have a very short wavelength, they have different characteristics than visible light. The most interesting characteristic of x-rays is their ability to pass through many materials. But the detection of x-rays is similar to that of visible light.

They easily pass through soft body tissues, but they are somewhat blocked by hard material like bones.

X-ray of a person's head

X-rays are almost completely stopped by lead. That is why lead shielding is used to protect people and things from excessive exposure to x-rays.

An important characteristic of x-rays is that they will expose photographic film, even if it is in a container. That is why you do not want to put a camera with film through an airport x-ray machine.

Thus, if a beam of x-rays passes through your body and exposes some film, a faint outline of your soft tissues will be seen, but your bones will show up distinctly. If a sheet of lead was put between the source of the x-rays and the film, it would not be exposed.

A dental x-ray requires holding a piece of film behind the teeth

Medical and dental x-ray technicians often wear a badge that contains photographic film to monitor how much stray x-ray radiation they receive in their jobs. The film in the badge is checked periodically to see how much exposure was received.

X-rays can also be detected electronically, with a detector similar to that used in a digital camera to record visible light.

Uses of x-rays

There are a variety of uses of x-rays.

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.

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.

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.

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 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. 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.