An x-ray is a high energy electromagnetic wave that passes through many objects. They are used in medical diagnosis to take a picture of the insides of human bodies. There are two main classifications of x-rays: soft x-rays and hard x-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…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).”¹http://www.livescience.com/32344-what-are-x-rays.html