Physics A2 · Quantum physics. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
When electromagnetic radiation falls on a metal surface, electrons can be knocked out of the surface. These are photoelectrons. The radiation arrives as photons, each of energy hf. One photon gives all its energy to one electron.
An electron needs a minimum energy to escape from the surface. This is the work function Φ of the metal. If hf is less than Φ, no electrons leave, however bright the light is. The lowest frequency that works is the threshold frequency.
Any energy left over becomes kinetic energy of the electron. A brighter beam of the same frequency has more photons per second, so more electrons per second, but each photon still carries the same energy.
Students say that brighter light gives faster electrons. Intensity changes only the number of electrons emitted per second; the maximum kinetic energy changes only when the frequency changes.
Answer: About 2.6 × 10−19 J (1.6 eV).
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