How did Einstein’s photoelectric-effect explanation use Planck’s constant?

In 1905, Albert Einstein extended the work of Max Planck to explain the photoelectric effect by proposing that light consists of discrete packets of energy, or photons.12

Einstein utilized Planck’s constant ($h$) to define the energy ($E$) of each individual photon based on the frequency ($\nu$) of the radiation, expressed by the relationship $E=h\nu$. He hypothesized that when light strikes a metal surface, an atom absorbs a single photon. If the photon's energy is sufficient, it frees an electron from the metal.13

The energy required for an electron to escape the surface is known as the work function ($W$ or $\varphi$). Einstein’s photoelectric equation, $E_k=h\nu -\varphi$, demonstrates that the maximum kinetic energy ($E_k$) of the emitted electron is determined by the difference between the photon's energy and the work function. This model successfully explained why electron emission depends on the frequency of light rather than its intensity, as well as the existence of a threshold frequency below which no emission occurs.142