Student Features

The Electromagnetic Spectrum - X-Rays
Remember that when you listen to the radio, watch TV, or cook dinner in a microwave oven, you are using electromagnetic waves. Radio waves, television waves, and microwaves are all types of electromagnetic waves. They only differ from each other in wavelength. Waves in the electromagnetic spectrum vary in size from very long radio waves the size of buildings, to very short gamma-rays smaller than the size of the nucleus of an atom. The electromagnetic spectrum includes X-rays.

As the wavelengths of light decrease, they increase in energy. X-rays have smaller wavelengths and therefore higher energy. We usually talk about X-rays in terms of their energy rather than wavelength. This is partially because X-rays have very small wavelengths!

Roentgen's wife's hand
Roentgen's wife's hand
X-rays were first discovered in 1895 by Wilhelm Conrad Roentgen, a German scientist who found them quite by accident.

He took an X-ray photograph of his wife's hand which clearly revealed her wedding ring and her bones. The photograph electrified the general public and aroused great scientific interest in the new form of radiation. Roentgen called it "X" to indicate it was an unknown type of radiation.

The Earth's atmosphere is thick enough that virtually no X-rays are able to come through from outer space all the way to the Earth's surface. This is good for us but also bad for astronomy - we have to put X-ray telescopes and detectors on satellites! We cannot do X-ray astronomy from the ground.

How do we "see" using X-ray light?

What would it be like to see X-rays? Well, we wouldn't be able to see through people's clothes, no matter what the ads for X-ray glasses tell us! If we could see X-rays, we could see things that either give off X-rays or stop their transmission. Our eyes would be like the X-ray film used in hospitals or dentist's offices. X-ray film "sees" X-rays, like the ones that travel through your skin. It also sees shadows left by things that the X-rays can't travel through (like bones or metal).

When you get an X-ray taken at a hospital, X-ray sensitive film is put on one side of your body, and X-rays are shot through you. At a dentist, the film is put inside your mouth, on one side of your teeth, and X-rays are shot through your jaw, just like in this picture. It doesn't hurt at all - you can't feel X-rays.

X-ray of a tooth
X-ray of a tooth
Because your bones and teeth are dense and absorb more X-rays then your skin does, silhouettes of your bones or teeth are left on the X-ray film while your skin appears transparent. Metal absorbs even more X-rays - can you see the filling in the image of the tooth

When the Sun shines on us at a certain angle, our shadow is projected onto the ground. Similarly, when X-ray light shines on us, it goes through our skin, but allows shadows of our bones to be projected onto and captured by film.

This is an X-ray photo of a 1-year-old girl. Can you see the shadow of what she swallowed?

X-ray of one-year old showing a swallowed pin
X-ray of 1-year-old showing a pin that she swallowed

We use satellites with X-ray detectors on them to do X-ray astronomy. In astronomy, things that give off X-rays (like black holes) are like the dentist's X-ray machine, and the detector on the satellite is like the X-ray film.

What does X-ray light show us?

Many things in space give off X-rays, among them are black holes, neutron stars, binary star systems, supernova remnants, stars, the Sun, and even some comets!

The Earth glows in many kinds of light, including the energetic X-ray band. Actually, the Earth itself does not glow - only aurora produced high in the Earth's atmosphere. These aurora are caused by charged particles from the Sun.

X-ray image of Earth
X-ray image of Earth (Credit: Polar, PIXIE, NASA)
To the left is the first picture of the Earth in X-rays, taken in March, 1996 with the orbiting Polar satellite. The area of brightest X-ray emission is red. The energetic charged particles from the Sun that cause aurora also energize electrons in the Earth's magnetosphere. These electrons move along the Earth's magnetic field and eventually strike the Earth's ionosphere, causing the X-ray emission. These X-rays are not dangerous because they are absorbed by lower parts of the Earth's atmosphere.

Many things in deep space give off X-rays. Many stars are in binary star systems - which means that two stars orbit each other. When one of these stars is a black hole or a neutron star, material is pulled off the normal star. This materials spirals into the black hole or neutron star and heats up to very high temperatures. When something is heated to over a million degrees, it will give off X-rays

Learn more about X-rays!