The Electron Diffraction is about the wave-like behavior of electrons. This was observed when electrons go through a grating and produce an interference pattern.

The Heisenberg's Uncertainty Principle is about the measurements of the variables of an electron. This states that the position and velocity of an electron are never known.

The Photoelectric Effect is about the emission of electrons due to the absorption of electromagnetic radiation. This occurs when light hits a piece of metal and electrons are ejected.

The Atomic emission spectra is about the discrete energy levels of electrons. This states that energy levels in an atom are quantized. Thus, electrons are found on distinct energy levels.