The basic science
Part of the electromagnetic spectrum, with wavelengths ranging from 10-8 metres to around 10-11 metres. (Or, if we are talking particle energies, 100 to 100 000 eV).
X-rays fit in the spectrum between ultra-violet radiation and gamma rays
So-called `soft' x-rays are in the lower energy (longer wavelength) part of this range, while hard x-rays are in the shorter wavelengths (and hence higher energies).
Medical x-rays are usually around 10-10 metres.
In general, X-rays are the highest energy photons which can still be focussed and bent (albeit with great difficulty) to make images. They are also of wavelengths comparable with atomic dimensions, and so can be used to investigate the composition and structure of atomic lattices (crystals). Furthermore, because of their small size and high energy, they can pass through many low-density materials, unimpeded, so are useful for viewing internal structures of opaque (to visible light) objects.
The history
On November 8, 1895, Dr Wilhelm Conrad Röntgen (1845-1923) was messing around with some cathode ray equipment in his laboratory in Wurzburg, Germany. He knew that cathode rays (an electron beam) travel in pre-determined directions and are quickly stopped by air, so he was surprised to see some fluorescent material across the lab start glowing when the equipment was switched on. He deduced that the fluorescence was caused by a new kind of ray or energy.
This was the first time X-rays were observed and documented by a scientist. Röntgen then spent the next few weeks investigating these rays.
He published his work in a paper submitted to the Wurzburg Physico-Medical Society on December 28, 1895, calling the rays X, for unknown. His colleagues later tried unsuccessfully--and against Röntgen's wishes--to call them Röntgen rays, although German speakers still use the word Röntgenstrahlen (thanks). Röntgen was awarded the Nobel prize for Physics in 1901 in recognition of this discovery.
It is famously said that Lord Kelvin dismissed the phenomenon as an elaborate hoax. This appears to be true, but it was in a letter written before Kelvin had seen Röntgen's experimental notes, because of illness. Once Kelvin saw the experimental notes and the evidence, he became a firm supporter of Röntgen and his X-rays.
One of the reasons for Kelvin's initial skepticism was that the turn of the century saw a great rise in mysticism: spiritualism, ectoplasm and the like. The ghostly pictures produced by Röntgen and his mysterious rays looked too much like faked ectoplasm pictures, so Kelvin initially dismissed it as yet another pseudo-scientific hoax.
The practical details
The following is taken from a translation of Röntgen's original paper:
If the discharge of a fairly large induction-coil be made to pass through a Hittorf vacuum-tube, or through a Lenard tube, a Crookes tube or other similar apparatus, which has been sufficiently exhausted, the tube being covered with thin, black cardboard which fits it with tolerable closeness, and if the whole apparatus be placed in a completely darkened room, there is observed at each discharge a bright illumination of a paper screen covered with barium platinocyanide, placed in the vicinity of the induction-coil, the fluorescence thus produced being entirely independent of the fact whether the coated or the plain surface is turned towards the discharge tube. This fluorescence is visible even when the paper screen is at a distance of 2 meters from the apparatus.
A more up-to-date approach to making X-rays for cancer and pleasure can be found here
http://xray.uu.se/hypertext/VacNews2.html
Seriously, though, Röntgen died from radiation exposure because he spent too much time playing with these things. Be careful, watch your dosage.
Optics
Focussing X-rays is not easy. There is no known material which will refract X-rays by any significant amount, so it is impossible to create a conventional lens. However, alternatives to the refracting lens do exist. A beam of X-rays will reflect if it meets a dense material at a shallow angle (less than 5 degrees).
For space-based X-ray telescopes, physicists have constructed lenses made from concentric cylindrical mirrors made of polished copper. X-rays entering the `lens' encounter one of the cylinders at a low glancing angle, and are reflected toward the focus. The outermost cylinders are designed to turn the beam through a larger angle than the inner cylinders, so creating a type of lens.
Applications
X-ray crystallography
X-ray Diffraction
X-ray astronomy
Medical x-rays
Oh, all right, then, and the Shoe-fitting fluoroscopeThanks, BD
Sources
http://xray.uu.se/
http://www.lbl.gov/MicroWorlds/ALSTool/EMSpec/EMSpec2.html
http://www.uic.edu/depts/mcne/founders/page0081.html
http://www-cxro.lbl.gov/multilayer/
http://www.cc.emory.edu/X-RAYS/century_05.htm
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