X-ray X-ray is invisible, highly penetrating electromagnetic radiation of much shorter wavelength(higher frequency) than visible light. The wavelength range for X rays is from less than a billionth of an inch to less than a trillionth of an inchX-ray X-ray is invisible, highly penetrating electromagnetic radiation of much shorter wavelength (higher frequency) than visible light. The wavelength range for X rays is from less than a billionth of an inch to less than a trillionth of an inch