The photoelectric effect is the emission of electrons by substances, especially metals, when electromagnetic waves (eg. light) of a certain frequency – a frequency that is higher than a threshold value– hit their surface.

If the electromagnetic radiation has a frequency below that of the threshold, the electrons are unable to gain sufficient energy to overcome the electrostatic barrier of the work function of the metal. Each type of surface and material has a unique threshold.

Figure: A simple representation of the photoelectric effect.