How did Albert Einstein’s photon theory explain light?

In 1905, Albert Einstein proposed a revolutionary theory that challenged the classical view of light as purely an electromagnetic wave. By extending Max Planck’s earlier work, Einstein argued that light is granular, consisting of discrete packets of energy called photons.12

Einstein’s theory explained the photoelectric effect through two key assumptions:

• Quantized Energy: Each photon carries a fixed amount of energy, $E=hf$, where $h$ is Planck’s constant and $f$ is the frequency of the light.13
• Discrete Absorption: An atom in a metal surface absorbs either a whole photon or none at all.1

When a photon strikes a metal surface, it transfers its energy to an electron. If the photon's energy exceeds the work function—the minimum energy required for an electron to escape the metal—the electron is emitted. The remaining energy becomes the electron's kinetic energy. This model successfully explained why electron emission depends on light frequency rather than intensity, a phenomenon that classical physics could not account for.1324