What are X-rays?

While some might think the answer is simple, it's not. We asked Daniel Mittleman, a professor at Rice University’s Department of Electrical and Computer Engineering, for the full explanation. Here's what he said:

X-rays are a form of electromagnetic radiation, just like visible light, infrared radiation, microwaves and radio waves. But compared to these other types of radiation, X-rays are more energetic.

An X-ray photon can be hundreds or thousands of times more energetic than a photon of visible light.

Wilhelm Roentgen first described X-rays in 1895, an achievement for which he was awarded the very first Nobel Prize in Physics. During World War I, X-rays were already being used for medical purposes.

Most of the X-rays in the universe arise when highly excited atoms decay back to their ground state configuration. For example, if an electron is removed from the inner shell orbitals of an atom—perhaps by a collision with something—then the atom will emit an X-ray photon as it returns to its equilibrium state.

Another common source of X-rays is from a process called bremsstrahlung, which is German for "braking radiation.” X-rays are emitted when a highly energetic beam of charged particles such as electrons is rapidly decelerated—because it runs into a metal target, for example.

In medical X-ray machines, a beam of energetic electrons is focused onto a target, usually a piece of tungsten. As the electrons are decelerated, this generates bremsstrahlung X-rays. In addition, the incoming electrons can collide with a tungsten atom and knock an electron out from its inner orbit. This kind of device actually produces X-rays by both mechanisms simultaneously.

Finally, a synchrotron machine that accelerates electrons in a circular orbit can produce X-rays. As the electrons spin around, they emit powerful beams of X-rays, which can be used for many different kinds of scientific research. There are a number of these machines around the United States and around the world, including the Advanced Light Source in Berkeley Calif., the Advanced Photon Source in Argonne Ill., and the National Synchrotron Light Source in Brookhaven, N.Y.

• Image Gallery: The Electromagnetic Spectrum
• The Enduring Mystery of Light
• Image Gallery: How Light Works