For a given material, there is a minimum frequency of light (the threshold frequency) below which no photoemission occurs

, regardless of the intensity of the incident light.

Electrons are only emitted if the frequency of the incoming light is at or above the threshold frequency.

The energy of a photon of light is directly proportional to its frequency, as described by the equation:

E = h * f

Where:

• E is the energy of the photon.
• h is Planck's constant.
• f is the frequency of the light.

When the energy of an individual photon is greater than or equal to the work function of the material (the energy needed to overcome the binding forces holding the electrons in place),

electrons can be emitted. The threshold frequency is the frequency at which the energy of the photon matches the work function, providing just enough energy to release electrons.

Below this frequency, the individual photons do not have enough energy to cause photoemission.