What is the work function of a metal?

Think of it as an "escape energy" for electrons. In simple terms, the work function of a metal is the minimum amount of energy needed to knock an electron completely free from its surface.

This is a key concept in the photoelectric effect. When light, which is made of photons, hits the metal, it transfers energy. If a single photon has more energy than the work function, it can liberate an electron. Any extra energy from that photon is then converted into the electron's kinetic energy, making it fly off faster.

If the photon's energy is less than the work function, nothing happens. The electron just doesn't have enough of a kick to escape. Every metal has its own unique value for this.