The Davisson-Germer experiment, conducted by Clinton Davisson and Lester Germer in 1927, provided experimental evidence for the wave-like behavior of electrons. They observed that electrons scattered from a crystal surface exhibited diffraction patterns, similar to those observed for X-rays, confirming the de Broglie hypothesis and supporting the wave-particle duality of matter.

The photoelectric effect is a phenomenon in which electrons are ejected from a material’s surface when it is exposed to light (usually ultraviolet or higher frequency). The effect cannot be explained by classical wave theory but is consistent with the particle-like behavior of light proposed by Albert Einstein, who described light as quanta of energy called photons.

Superfluidity is a quantum mechanical phenomenon observed in certain liquids, such as helium-4, at temperatures close to absolute zero. In a superfluid, particles flow without experiencing any viscosity or resistance to flow, exhibiting frictionless motion and unique properties such as the ability to flow through capillaries and climb up container walls.

The Pauli exclusion principle, formulated by Wolfgang Pauli, is a fundamental principle of quantum mechanics that states that no two fermions (particles with half-integer spin) can occupy the same quantum state simultaneously. This principle underlies the stability of matter and the electronic structure of atoms, leading to the organization of electrons into distinct energy levels and the formation of chemical bonds.

The Bohr model of the atom, proposed by Niels Bohr in 1913, is a simplified model that describes the structure of the hydrogen atom. It postulates that electrons orbit the nucleus in discrete energy levels or orbits, and can only transition between these orbits by absorbing or emitting specific amounts of energy in the form of photons. The model successfully explained the spectral lines of hydrogen but was later superseded by quantum mechanics.