Franck-Hertz Experiment: Evidence of Quantum Mechanics

by Yuvi K - December 26, 2023

Franck-Hertz Experiment: Evidence of Quantum Mechanics

Introduction

The Franck-Hertz experiment, conducted in 1914 by James Franck and Gustav Hertz, stands as a cornerstone in the realm of quantum mechanics. This groundbreaking experiment provided compelling evidence for the quantization of energy levels in atoms, a fundamental concept in quantum physics. The results of the Franck-Hertz experiment reshaped our understanding of the atomic world and laid the foundation for numerous technological advancements.

Background

Before delving into the experiment itself, it’s essential to grasp the scientific context of the early 20th century. At that time, the classical model of the atom, often attributed to Niels Bohr, described electrons orbiting the nucleus in fixed energy levels or orbits. However, this model faced challenges in explaining certain phenomena, such as the emission and absorption spectra of atoms.

The Experiment

The Franck-Hertz experiment was designed to investigate the behavior of electrons in a vacuum when subjected to varying levels of energy. The setup consisted of a vacuum tube containing mercury vapor and a series of electrodes. Electrons emitted from a heated cathode were accelerated toward a positively charged anode.

Here’s a step-by-step overview of the experiment:

1. Electron Emission: The cathode, heated to a high temperature, emitted electrons. These electrons were accelerated through a voltage gradient as they moved toward the anode.
2. Collision with Mercury Atoms: As the accelerated electrons traveled through the mercury vapor, they collided with mercury atoms. During these collisions, some electrons transferred energy to the mercury atoms, causing them to become excited.
3. Energy Levels in Mercury Atoms: The excited mercury atoms were raised to higher energy levels. These energy levels were quantized, meaning the atoms could only absorb specific discrete amounts of energy.
4. Energy Conservation: To reach higher energy levels, electrons had to possess sufficient energy. Electrons with energy below a certain threshold were unable to excite the mercury atoms.
5. Observing the Phenomenon: The experimenters measured the current passing through the tube as a function of the applied voltage. They observed distinct regions where the current decreased significantly, indicating that electrons were losing energy during collisions with mercury atoms.

Key Observations and Implications

The crucial observations from the Franck-Hertz experiment were as follows:

1. Quantization of Energy Levels: Electrons could only transfer discrete amounts of energy to the mercury atoms, demonstrating the quantization of energy levels in atoms. This was a direct challenge to classical physics.
2. Threshold Voltage: The experiment revealed a threshold voltage below which electrons lacked the energy required to excite mercury atoms. Once the electrons surpassed this threshold, they could excite the atoms.
3. Current Minima: The distinct regions of decreased current, known as minima, corresponded to the points where electrons lost specific amounts of energy. These minima were evidence of quantized energy levels in the mercury atoms.

Impact on Quantum Mechanics

The Franck-Hertz experiment provided empirical evidence that electrons could only exist in quantized energy levels within atoms. This discovery revolutionized the field of quantum mechanics and led to a deeper understanding of atomic and subatomic physics. Niels Bohr later incorporated these ideas into his atomic model, which successfully explained the spectral lines of hydrogen.

Applications and Legacy

The Franck-Hertz experiment’s legacy extends beyond the realm of fundamental physics. It laid the groundwork for the development of technologies like electron microscopy and quantum computing. Moreover, it exemplifies the power of experimentation in uncovering the hidden truths of the quantum world.

In conclusion, the Franck-Hertz experiment stands as a testament to the wonders of quantum mechanics. It provided unequivocal evidence for the quantization of energy levels within atoms, challenging classical physics and reshaping our understanding of the atomic world. This groundbreaking experiment continues to inspire scientists and engineers in their quest to unravel the mysteries of the quantum realm.