The photoelectric effect marks the beginning of modern physics and the shift from classical to quantum theory. It occurs when light shining on a metal surface causes electrons to be emitted. Classical wave theory could not explain this phenomenon, because according to it, light intensity should determine how much energy the electrons receive. However, experiments showed that only light above a certain frequency can cause emission, regardless of intensity.
Albert Einstein explained the effect by proposing that light consists of discrete packets of energy called photons. Each photon has energy equal to Planck’s constant times its frequency (E = hf). When a photon strikes the metal surface, it transfers its energy to an electron. If the energy is greater than the work function of the metal — the minimum energy needed to release an electron — the electron is emitted.
This discovery showed that energy is quantized and that light has both wave and particle properties. The photoelectric effect provided crucial evidence for quantum theory and earned Einstein the Nobel Prize in Physics in 1921.
Understanding the photoelectric effect helps A-Level students connect classical ideas of waves and particles to quantum mechanics. It also lays the groundwork for later topics like atomic structure, electron energy levels, and modern technologies such as solar cells and photodiodes.