Why Do Emission and Absorption Lines Occur?
According to the laws of Quantum Mechanics, there are only a specific number of energy states in which a given atom, ion, or molecule can exist. [Quantum Mechanics is a branch of physics which is able to predict the consequences of electromagnetic radiation having both wave-like and particle-like properties. Note that a bundle of energy at one specific wavelength, i.e., a quantum, is also called a photon.] Spectral lines arise from changes in the amounts of energy present in the atoms, ions, or molecules that cause them. The allowed energy states of an atom, ion, or molecule are discrete (i.e., separate and distinct); they are quantized. When atoms, ions, or molecules lose (emit) photons of the same discrete energy, an emission line is produced in the background continuum source. When atoms, ions, or molecules gain (absorb) photons of the same discrete energy (provided by a background continuum source), an absorption line is produced. The Hydrogen Spectrum: The Simplest Example The neu
