Photoelectric effect is the effect of the emission of electrons from a metal surface if the photon energy exceeds the work function of the metal. It is a key discovery in physics for the particle (photon) model of light as the wave model could not explain the presence of a threshold frequency and the proportionaility of the number of electrons emitted to the intensity.
Photons are emitted from atoms due to the excitation of electrons from a lower enegry state to a higher energy state and following up by a deexcitation and hence emitted a photon with energy equal to the energy gap. Line spectra can be used as evidence for transitions between discrete quantised energy levels.
Particles can act as waves and waves and act like particles. Evidence for wave behaviour can be seen through the Young's Double Slit experiment and electron diffraction; evidence for particle behaviour can be seen from the photoelectric effect and emission spectra. De Broglie hypothesised that particles each have an associated de Broglie wavelength inversely proportional to its momentum.