Date: 16 May 2012
Depicts: The photoelectric effect
Copyright: ESA / AOES Medialab

Light rays striking a surface will be absorbed if their energy is higher than a certain threshold value, which depends on the surface material. The energy of the absorbed light is transferred to electrons in the material, which are then emitted. This phenomenon, known as the photoelectric effect, generally occurs with highly energetic radiation such as ultraviolet light, X- and gamma rays. For a dramatic way to teach the subject at school, see Bernardelli (2010).

Two other phenomena may take place when highly energetic radiation interacts with matter: the Compton effect, in the case of the most energetic X-rays and for gamma rays, and pair production, only for gamma rays. Depending on the surface material and on the energy of the radiation, these phenomena can be more likely to happen than the photoelectric effect.