X-rays are a form of invisible, high-frequency electromagnetic radiation. The wavelength of x-rays is between 10 and 0.01 nanometers, corresponding to a frequency of 30 PHz to 30 EHz. X-rays are very small and energetic. They are produced by accelerating electrons at a metal target. X-rays are used in various medical applications, especially for imaging.

Because of the tiny wavelength of x-rays, they are useful for imaging small structures, such as individual molecules. This research field is known as x-ray crystallography, which was responsible for uncovering the physical structure of DNA, among other achievements. Today it is frequently used to image the molecular structure of complex proteins.

Sometimes x-ray imaging is used to investigate paintings which have been painted over. Their most prominent use is probably to image bones to discover whether they are actually broken or not. X-rays are a carcinogen, so use of them for medical purposes should be kept brief

Under special circumstances, such as in a dark room and looking directly down a cathode ray tube, x-rays are visible to the human eye. It is not known whether this is the eye directly perceiving the rays, or from a secondary effect such as the creation of luminescent sparks in the eyeball. In any case, this fact about x-rays is not widely publicized, likely because it would be dangerous to test it very extensively.