Franck–Hertz experiment

Tiny bits of energy fly through a glass tube.

Inside, they hit small pieces of metal. Some bits move very slowly. They bounce off the metal without losing speed.

Other bits move much faster. When they hit the metal, they lose a lot of speed. This happens because the metal takes their energy. The metal then lets out a tiny flash of light. This shows how small parts of our world work. It is a very big discovery!

In 1914, James Franck and Gustav Hertz did a big test. They used a glass tube filled with mercury vapor.

They sent tiny bits called electrons flying through the tube. These electrons hit the mercury atoms.

Sometimes the electrons hit an atom and just bounce off. This is an elastic collision. The electron changes direction but keeps its speed.

Other times, the electrons hit an atom much harder. This is an inelastic collision. The electron gives 4.9 electron volts of power to the atom. An electron volt is a tiny unit of energy. When this happens, the electron slows down a lot.

The mercury atom takes that power and gets excited. Soon, the atom lets out a flash of ultraviolet light.

This light has a specific wavelength.

This test proved the Bohr model was right. Niels Bohr said electrons live in specific energy levels. They cannot stay between these levels. The electrons in the mercury atoms jumped from a low level to a higher one. This work won the Nobel Prize in Physics in 1925.

The Franck–Hertz experiment is a famous test in science. It showed us how atoms actually work.

Before this test, people were not sure about the tiny parts inside an atom. This experiment proved that energy in an atom comes in specific amounts. We call these amounts "quanta." This discovery helped build the field of quantum mechanics. It changed how we see the whole physical world.

To see this, scientists used a special glass tube. Inside the tube, there was a thin vapor of mercury atoms.

They sent tiny particles called electrons flying through the mercury. There are two ways these electrons hit the atoms. Sometimes, they have an elastic collision. This means the electron bounces off but keeps its speed. Other times, they have an inelastic collision. In this case, the electron hits the atom very hard. It gives exactly 4.9 electron volts of energy to the mercury atom. This makes the electron slow down a lot.

James Franck and Gustav Hertz performed this work in 1914. They presented their findings to the German Physical Society in April.

They used a heated vacuum tube at 115 degrees Celsius. This temperature kept the mercury vapor at about 100 pascals. Their results matched a new idea from Niels Bohr. Bohr had proposed a model for the atom just one year earlier. This model suggested that electrons live in specific energy levels. They cannot exist in the spaces between these levels.

When the mercury atom gets that 4.9 electron volts of energy, it becomes excited.

A short time later, the atom releases that energy as light. This light is ultraviolet light with a wavelength of 254 nanometers. Franck and Hertz also measured the electric current in the tube. They saw the current drop sharply at 4.9 volts. It dropped again at 9.8 volts because of a second collision. This pattern of drops continued at every 4.9 volt step. These numbers proved the energy levels were real and fixed.

This experiment links to how we understand light and energy today. It confirmed the work of many great scientists. Niels Bohr used ideas from Hendrik Lorentz to build his model. Even Albert Einstein was impressed by these results. He once said the discovery was so lovely it makes you cry. Because of this amazing work, Franck and Hertz won the Nobel Prize in Physics in 1925. They showed us that the tiny world of atoms follows very strict rules.

The Franck–Hertz experiment is a landmark in the history of physics. It provided the first electrical measurement that clearly demonstrated the quantum nature of atoms.

Before this discovery, scientists were still debating how energy worked inside an atom. This experiment proved that atoms do not absorb energy in any random amount. Instead, they only accept specific, fixed amounts of energy. This finding confirmed that the world of the very small follows unique rules. It helped move science from classical ideas toward the field of quantum mechanics.

To perform the experiment, James Franck and Gustav Hertz used a specialized vacuum tube.

This tube contained a thin vapor of mercury atoms. The researchers heated the tube to 115 degrees Celsius. At this temperature, the mercury vapor pressure was about 100 pascals. Inside the tube, they used three main parts: a hot cathode, a metal mesh grid, and an anode. The cathode emits electrons, which are tiny particles with a negative charge. By applying voltage, the scientists could control the speed and energy of these flying electrons.

As the electrons travel through the mercury vapor, they experience two different types of collisions.

The first type is an elastic collision. This happens when a slow-moving electron hits a mercury atom. In an elastic collision, the electron changes direction, but it does not lose much speed or kinetic energy. This occurs because the mercury atom is about 400,000 times more massive than the electron. The second type is an inelastic collision. This happens only when an electron reaches a specific speed of about 1.3 million meters per second. At this speed, the electron has exactly 4.9 electron volts (eV) of kinetic energy.

In an inelastic collision, the electron transfers exactly 4.9 eV of energy to the mercury atom.

This transfer causes the electron to slow down significantly. The mercury atom becomes "excited" because it has absorbed this specific amount of energy. The electron inside the mercury atom moves from its lowest energy level to a higher energy level. Because the electron is now more loosely bound to the atom, it stays in this higher state for a short time. Eventually, the atom returns to its original state by releasing that energy as ultraviolet light. This light has a very specific wavelength of 254 nanometers.

Franck and Hertz measured these collisions by tracking the electric current in the tube.

They observed that the current increased steadily as they raised the voltage. However, when the voltage reached 4.9 volts, the current dropped sharply. This drop happened because the electrons lost so much energy in collisions that they could no longer reach the anode. If they increased the voltage further, the current rose again. At 9.8 volts, the current dropped a second time. This pattern repeats at every 4.9-volt interval, such as 14.7 volts or 19.6 volts. Each drop represents electrons undergoing an additional inelastic collision.

These results were revolutionary because they supported the Bohr model of the atom.

Proposed by Niels Bohr in 1913, this model suggested that electrons occupy specific "quantum energy levels." Bohr's model was a precursor to modern electron shell models. It stated that electrons cannot exist in the spaces between these levels. The Franck–Hertz experiment proved that these levels were real and fixed. This was a major change from previous expectations, which suggested electrons could be bound by any amount of energy. Bohr had built his model using ideas from Hendrik Lorentz regarding the quantization of energy.

The significance of this work cannot be overstated. The discovery of the laws governing how electrons impact atoms earned Franck and Hertz the Nobel Prize in Physics in 1925.

Even Albert Einstein was moved by the elegance of the results, famously remarking that the discovery was "so lovely it makes you cry." The experiment successfully linked the movement of particles to the emission of light. It bridged the gap between electricity and the study of light spectra. By proving the existence of discrete energy levels, it helped solidify the foundation of modern atomic physics.