However fiercely red light shines on a zinc plate, not one electron leaves; a feeble violet beam ejects them immediately. Intensity only adds more photons, but beating the work function W₀ requires each individual photon to carry enough energy hf. So the maximum kinetic energy of the ejected electrons tracks the colour while the photocurrent tracks the brightness, and the two come apart completely. The threshold moves with the metal: caesium needs only 2.10 eV, so yellow-green will do, while platinum demands 5.65 eV and only ultraviolet works.
However fiercely red light shines on a zinc plate, not one electron leaves; a feeble violet beam ejects them immediately. Intensity only adds more photons, but beating the work function W₀ requires each individual photon to carry enough energy hf. So the maximum kinetic energy of the ejected electrons tracks the colour while the photocurrent tracks the brightness, and the two come apart completely. The threshold moves with the metal: caesium needs only 2.10 eV, so yellow-green will do, while platinum demands 5.65 eV and only ultraviolet works.
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