Editing Electromagnetic radiation (section)

=== Wave–particle duality ===
{{Main|Wave–particle duality}}

The modern theory that explains the nature of light includes the notion of wave–particle duality. The theory is based on the concept that every quantum entity can show wave-like or particle-like behaviors, depending on observation. The observation led to the collapse of the entity's [[wave function]]. If it is based on the [[Copenhagen interpretation]], the observation does really collapse the wave function; for the [[many-worlds interpretation]], all possible outcomes of the collapse happened in [[Multiverse|parallel universes]]; for the [[pilot wave theory]], the particle behaviour is simply determined by waves. The duality nature of a real photon has been observed in the [[double-slit experiment]].

Together, wave and particle effects fully explain the emission and absorption spectra of EM radiation. The matter-composition of the medium through which the light travels determines the nature of the absorption and emission spectrum. These bands correspond to the allowed energy levels in the atoms. Dark bands in the [[absorption spectroscopy|absorption spectrum]] are due to the atoms in an intervening medium between source and observer. The atoms absorb certain frequencies of the light between emitter and detector/eye, then emit them in all directions. A dark band appears to the detector, due to the radiation scattered out of the [[light beam]]. For instance, dark bands in the light emitted by a distant [[star]] are due to the atoms in the star's atmosphere.