The photoelectric effect occurs when light shines on a metal surface, causing the metal to emit electrons. This phenomenon provided crucial evidence for the particle nature of light and the development of quantum mechanics.

The Classical Problem

According to classical wave theory, light is a continuous wave of electromagnetic energy. If you shine a bright enough light (high intensity) on a metal for a long enough time, the electrons should eventually absorb enough energy to escape, regardless of the light's frequency (color). However, experiments showed that if the light's frequency was below a certain threshold, no electrons were emitted, no matter how bright the light was or how long it shone.

Einstein's Quantum Solution

In 1905, Albert Einstein resolved this by proposing that light consists of discrete packets of energy called "quanta" or "photons." The energy of a photon is proportional to its frequency, given by the equation $E = hf$, where $h$ is Planck's constant and $f$ is the frequency.

An electron can only be emitted if it absorbs a single photon with enough energy to overcome the forces holding it to the metal (the work function, $\Phi$). If $hf < \Phi$, a single photon doesn't have enough energy, and the electron cannot escape, regardless of the total number of photons (brightness). If $hf > \Phi$, the electron is ejected, and any excess energy becomes the electron's kinetic energy: $K_{max} = hf - \Phi$.