November 5, 2009

On this day in 1895, X-rays were discovered!

Wilhelm Conrad Roentgen is credited with the discovery and naming of X-rays. He called the new, invisible ray “X” to stand for unknown. However, in some countries (including Roentgen's native Germany), X-rays are called Roentgen rays. (He strongly objected.) Whatever we call them, these “rays” are a form of electromagnetic radiation, like light, ultraviolet light, and microwaves.

The first picture of a human body part ever taken with X-rays is the hand of Roentgen's wife. Roentgen published his scientific paper on the new form of radiation in December, 1895, and he showed other scientists the X-ray of the hand; the medical benefit of seeing through flesh to bone or metal was immediately apparent, and X-rays began to be used for broken bones and gunshot wounds just a few months later, in early 1896. American inventor Thomas Edison came up with a fluoroscope that became widely used to take X-rays. (Edison gave up X-ray research, however, when one of his glassblowers got cancer. We now know that energetic forms of electromagnetic radiation can be dangerous, and we are careful to shield X-ray technicians and patients from exposure as much as possible.)

Quick Quiz
1. Which of these, like X-rays, is a form of electromagnetic
Radio waves Water waves Sound waves

2. How fast do X-rays travel?
Instantly Speed of sound Speed of light

3. Which is the best shield against X-rays?
Water Wood Lead

4. Which emits the most X-rays?
Ordinary stars Black holes Gas planets

5. X-rays are like gamma rays in that both are
high-frequency, short-wavelength forms of
radiation. X-rays come from...
electrons. nuclei. neutrons.


1) radio waves 2) speed of light 3) lead 4) black holes 5) electrons

How X-Ray Machines Work

This diagram shows a very simplified version of an X-ray machine. The X-rays are able to travel through the skin, muscle, and hospital gown and hit the fluorescent screens, and this exposure makes this part of the screen light. However, the X-rays are to a great extent blocked by bone or metal, so those areas of the X-ray picture remain dark.

You can do a safe demonstration analogous to taking an X-ray picture by putting a window screen over a box or clean patch of pavement. Put a cardboard shape on top of the screen, then carefully scatter sand over the entire box or screened area. When the cardboard and screen are removed, the area that had been blocked by the cardboard is almost entirely free of sand, and this area makes a kind of negative “picture” of the cardboard shape.


X-rays are used to determine the arrangement of atoms in a solid such as a protein. Check out the beautiful images available when you type “Crystallography” into Google Image search.

Credit: the U.S. Department of Energy Human Genome Program

Extra: on Credit

Roentgen is credited with the discovery of X-rays, but he is not the first person to notice this form of radiation; he is the first to systematically study it and publish a paper on it, and he is certainly the one that named this radiation X-rays. He made his discovery by accident when using a cathode ray tube in his lab. Other scientists who worked with cathode ray tubes, since the invention of Crooke's tubes in 1875, had noticed the effect of some invisible energy on such things as photographic plates. Some did not investigate at all, and others, such as Nikola Tesla, did careful experiments but did not end up creating a catchy name and a medical demand for the phenomenon. Roentgen earned a Nobel Prize for his work with X-rays.

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