A Self-Guided Tour of the Electromagnetic Spectrum
When Wilhelm Roentgen discovered X-rays in 1895, he didn't understand what they were, so he called them "X-rays." We now know that X-rays are electromagnetic waves that occupy the spectrum between ultraviolet and gamma radiation.
X-rays have myriad uses - everything from examining a person's bones to inspecting computer chips, from analyzing the structure of crystals to photographing celestial objects.
The X-ray radiograph is an invaluable tool in the field of medicine, allowing doctors to see bone and organs within the body as well as tumors, dental cavities, and other undesirables.
X-ray radiation is also used in the treatment of cancer, as it kills faster-growing cancer cells more readily than normal cells.
The extremely hot clouds of gas that surround supernova explosions are a good source of X-ray radiation. Here's an X-ray image of the supernova remnant Cassiopeia A. In this image, different X-ray wavelengths have been assigned different colors: blue = high energy wavelengths, green = medium energy, and red = low energy.