Kathie Couri, age 10, of Peoria, Illinois, for her question

How did X rays get their name?

The letter X gets a lot of use in math problems. It stands for the unknown item in an equation and you must work out the puzzle to learn what it really is. Way back in 1895, a  scientist discovered some invisible rays of energy that penetrate through certain solid objects. He did not know what they were. So he recalled the mysterious X used in math problems   and called his mysterious discovery X rays.

The whole universe teems with invisible energies, such as radio and gravity, magnet¬ism and electricity. Light and radio are forms of electromagnetic energy    related to both magnetism and electricity. They fan out in straight lines at more than 11 million miles per minute. And they pulse along in tiny bursts of energy called wavelengths. These wavelengths can be compared to the waves of the sea and their lengths are measured from one crest .to the next.

In an ordinary sunbeam there are countless longer and shorter wavelengths of pulsing energy. When each one is separated from the sunbeam, it sheds a colored ray of a certain color. When all the wavelengths of light are separated, they reveal the colors of the rainbow. There are seven main colors with many, many shaded tones between them. But all the colors are arranged strictly in order of their wavelengths. The shortest are the blue rays  the longest are red.

Light is but a small portion of all the electromagnetic energy around us. Invisible ultraviolet rays are slightly shorter than the shortest blue rays of visible light. Like the wavelengths of light, they are stopped by solid objects. But in 1895, other wave¬lengths were found that passed through solid flesh and revealed the bones buried inside the body.. Wilhelm Roentgen, the German scientist who discovered them, was mystified. He called them X rays because he did not know what else to call them. Most of us still call them X rays, though many scientists call them roentgen rays in honor of the mystified scientist who first discovered them.

We now know that X rays are electromagnetic energy with wavelengths shorter than those of ordinary light. Those used by doctors and dentists are called soft X rays. They pierce only through soft fleshy parts of the body and cannot pass through bones. Those used in industry have even shorter wavelengths. They are called hard X rays because they can reveal flaws hidden inside dense metals. All X rays work because their very short pulses of energy can wriggle around the tiny atoms that stop the longer wavelengths of ordinary light.

When the doctor turns on his X ray machine, the rays pass through the body onto a photographic film. These soft X rays show the bones as dark shadows because they cannot pass through them. Hard X rays can pierce through dense metals and reveal the delicate crystal structure of minerals. Each substance has its own crystal arrangement and an X ray picture is somewhat like a fingerprint identification.