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When coherent waves overlap, their amplitudes combine. Depending on phase, the result can be reinforcement, cancellation, or anything between. Interference turns phase differences into observable intensity patterns and lies behind interferometers, thin-film colours and many precision measurements. Diffraction is the spreading and restructuring of a wave when it encounters an aperture or obstacle whose dimensions are relevant to its wavelength. It is not a defect of waves: it is one of their signatures, and it ultimately limits the resolving power of optical instruments. In the photoelectric effect, electrons are emitted from a material when incident electromagnetic radiation has sufficiently high photon energy. Increasing intensity raises the number of available photons, but below the threshold frequency it does not compensate for insufficient photon energy.
Primary references
IUPAC Gold Book · NIST · BIPM