Photoelectric effect

Essential knowledge 15.5.A.1 defines the effect as the emission of electrons when electromagnetic radiation is incident on a photoactive material. One printed equation runs it, $K_{\max} = hf - \phi$, where $\phi$ is the work function of the material.

Two CED observations are what break the wave model.

• 15.5.A.2.i: light at or above the threshold frequency causes emission regardless of how many photons strike the material. Below that frequency, nothing is emitted however intense the beam.
• 15.5.A.2.ii: the energy of the emitted electrons does not depend on the number of incident photons, which the CED cites as evidence that light is a collection of discrete, quantized packets.

A wave delivering energy continuously predicts the opposite on both counts: enough intensity at any frequency should eventually free an electron, and a brighter beam should free faster ones. Neither happens. The classic third argument, that emission begins with no measurable delay, is not part of the AP Physics 2 required content.

Intensity does still do something. It sets how many electrons leave per second, so it sets the current, and it never touches $K_{\max}$ or the stopping potential.

Topic 15.5 owns the graph work and the experimental procedure.