This question is about the photoelectric effect.

In an experiment to investigate the photoelectric effect, light of frequency ƒ is incident on the metal surface A, shown in the diagram below. A potential difference is applied between A and B. The photoelectric current is measured by a sensitive ammeter. (Note: the complete electrical circuit is not shown.)

When the frequency of the light is reduced to a certain value, the current measured by the ammeter becomes zero. Explain how Einstein’s photoelectric theory accounts for this observation.

light consists of photons/quanta/packets of energy;
each photon has energy E=hf / photon energy depends on frequency;
a certain amount of energy is required to eject an electron from the metal;
if photon energy is less than this energy, no electrons are emitted;