Understanding Experiments in Atomic Physics

The question asks which specific experiment provided experimental evidence for the existence of discrete energy levels within atoms. This is a fundamental concept in quantum mechanics and atomic physics, explaining why atoms emit and absorb light at only specific wavelengths.

The Franck and Hertz Experiment and Discrete Energy Levels

The experiment that directly demonstrated the existence of discrete energy levels in atoms was performed by James Franck and Gustav Hertz in 1914. The goal of the Franck Hertz experiment was to study inelastic collisions between electrons and gas atoms (typically mercury vapor).

Here's a simplified breakdown of the experiment:

• Electrons were accelerated through a gas of atoms.
• The energy of the electrons was gradually increased.
• Initially, electrons lost very little energy in collisions with atoms (elastic collisions).
• However, when the electron energy reached a specific threshold value, they suddenly lost a significant amount of energy in collisions (inelastic collisions).
• This specific energy loss corresponded exactly to the energy difference between the atom's ground state and its first excited state.
• As the electron energy was increased further, other specific energy losses were observed, corresponding to excitation to higher atomic energy levels.

This experiment provided compelling experimental proof that atoms do not absorb energy continuously but only in discrete amounts, confirming the existence of specific, quantized discrete energy levels within the atom.

Examining Other Experiments

Let's look at why the other options, while significant in physics, did not directly demonstrate the existence of discrete energy levels:

• Davisson and Germer: The Davisson Germer experiment (1927) demonstrated the wave nature of electrons by observing electron diffraction patterns when electrons were scattered by a crystal lattice. This was a crucial confirmation of de Broglie's hypothesis but did not directly reveal the atomic energy levels within individual atoms.
• Planck: Max Planck (1900) introduced the concept of energy quantization in his explanation of blackbody radiation. He proposed that energy is emitted or absorbed in discrete packets (quanta), with energy \(E = h\nu\). While his work laid the foundation for quantum theory and the idea of energy quantization, it was a theoretical proposal for radiation, not a direct experimental demonstration of the quantized energy levels within an atom's structure itself.
• Rutherford: Ernest Rutherford's gold foil experiment (1911) led to the nuclear model of the atom, where a dense, positively charged nucleus is surrounded by electrons. The Rutherford model explained the scattering of alpha particles but could not explain the stability of atoms or the discrete nature of atomic spectra, which are consequences of discrete energy levels.

Therefore, the Franck and Hertz experiment stands out as the direct experimental evidence showing that atoms have discrete energy levels.