Metal can eject electron when light rays of particular frequency fall on it. Every metal surface has certain minimum frequency at which electrons are ejected from the metal surface. Below this minimum frequency no electrons are ejected from it, and this minimum frequency is known as Threshold Frequency.

Threshold Frequency is a characteristic frequency of a material.

The minimum frequency of the electromagnetic radiation which is required to eject the electrons from the surface of the metal is termed as the threshold frequency.

The emission of the electrons form the surface of the metals by the use of the light energy is known as the photoelectric effect.