X-rays are a form of electromagnetic radiation, as are radio waves, infrared radiation, visible light, ultraviolet radiation and microwaves. One of the most common and beneficial uses of X-rays is for medical imaging. X-rays are also used in treating cancer and in exploring the cosmos.
Electromagnetic radiation is transmitted in waves or particles at different wavelengths and frequencies. This broad range of wavelengths is known as the electromagnetic spectrum. The EM spectrum is generally divided into seven regions in order of decreasing wavelength and increasing energy and frequency. The common designations are: radio waves, microwaves, infrared (IR), visible light, ultraviolet (UV), X-rays and gamma-rays.
X-rays are roughly classified into two types: soft X-rays and hard X-rays. Soft X-rays fall in the range of the EM spectrum between (UV) light and gamma-rays. Soft X-rays have comparatively high frequencies — about 3 × 1016 cycles per second, or hertz, to about 1018 Hz — and relatively short wavelengths — about 10 nanometers (nm), or 4 × 10−7 inches, to about 100 picometers (pm), or 4 × 10−8 inches. (A nanometer is one-billionth of a meter; a picometer is one-trillionth of a meter.) Hard X-rays have frequencies of about 1018 Hz to higher than 1020 Hz and wavelengths of about 100 pm (4 × 10−9 inches) to about 1 pm (4 × 10−11 inches). Hard X-rays occupy the same region of the EM spectrum as gamma-rays. The only difference between them is their source: X-rays are produced by accelerating electrons, while gamma-rays are produced by atomic nuclei.