The threshold frequency is the minimum frequency of light that can cause the photoelectric effect, where light incident on a material causes electrons to be emitted. When the frequency of light is below the threshold frequency, no electrons are emitted, regardless of the intensity of the light. Einstein’s photoelectric equation, E = hf – Φ, relates the energy (E) of light, where h is Planck’s constant, f is the frequency of light, and Φ is the work function of the material, to the kinetic energy of emitted electrons.
