Threshold frequency
The minimum frequency of light required to eject an electron from a material in the photoelectric effect. It is related to the work function Φ by f₀ = Φ/h. Light with frequency below the threshold cannot eject electrons, regardless of intensity.
Formula
f₀ = Φ/h
Real World
Zinc has a threshold frequency in the ultraviolet range, so visible light from a desk lamp cannot eject electrons from it — but a UV lamp can, which is easily demonstrated in school physics labs.
Exam Focus
Clearly state that below threshold frequency no electrons are emitted regardless of intensity — a top discriminator mark.
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