when they are bombarded by electrons. The X-ray photons are emitted when the incident electrons knock an inner orbital electron out of an atom. When this happens an outer electron falls into the inner shell to replace it, losing potential energy (Δ‎E) in doing so. The wavelength λ‎ of the emitted photon will then be given by λ‎ = ch/Δ‎E, where c is the speed of light and h is the Planck constant.

X-rays can pass through many forms of matter and they are therefore used medically and industrially to examine internal structures. X-rays are produced for these purposes by an X-ray tube.