The Franck-Hertz experiment, conducted in 1914 by James Franck and Gustav Hertz, played a pivotal role in validating the quantization of atomic energy levels, a cornerstone of quantum mechanics. In this experiment, electrons, accelerated through a potential difference, collide with mercury vapor. The crucial finding was that electrons cause excitation in mercury atoms only when their energy matches the specific energy levels of the atoms. This provided tangible evidence for Niels Bohr’s theory of quantized energy levels in atoms. The Franck-Hertz experiment marked a significant step in the development of quantum mechanics, offering experimental support for the wave-particle duality of electrons and contributing to our understanding of the discrete nature of atomic energy states.