When the light from a hot gas is passed through a prism or diffraction grating, it forms a line spectrum: a set of sharp coloured lines at particular wavelengths on a dark background.Electrons in an atom can only occupy discrete energy levels. When an excited electron drops from a higher level E1 to a lower level E2, the atom emits a single photon whose energy equals the difference between the levels, hf = E1 - E2.
Because only certain energy differences are possible, only certain photon energies, and so only certain frequencies and wavelengths, are emitted. The energy of a photon is:
Each element has its own set of energy levels, so its line spectrum is unique. This is why line spectra are used to identify elements in stars and nebulae.If white light passes through a cool gas, the atoms absorb photons with exactly the energies needed to excite electrons. This produces an absorption spectrum of dark lines on a continuous spectrum, at the same wavelengths as the bright lines in that element's emission spectrum. Line spectra are evidence that atomic energy levels are discrete.
Because only certain energy differences are possible, only certain photon energies, and so only certain frequencies and wavelengths, are emitted. The energy of a photon is:Each element has its own set of energy levels, so its line spectrum is unique. This is why line spectra are used to identify elements in stars and nebulae.If white light passes through a cool gas, the atoms absorb photons with exactly the energies needed to excite electrons. This produces an absorption spectrum of dark lines on a continuous spectrum, at the same wavelengths as the bright lines in that element's emission spectrum. Line spectra are evidence that atomic energy levels are discrete.