X ray, invisible, highly penetrating electromagnetic radiation of much shorter wavelength (higher frequency) than visible light. The wavelength range for X rays is from about 10-8 m to about 10-11 m, or from less than a billionth of an inch to less than a trillionth of an inch; the corresponding frequency range is from about 3 × 1016 Hz to about 3 × 1019 Hz (1 Hz = 1 cps).
