Line Spectra

When the light from a hot, low-pressure gas (for example in a discharge tube) is passed through a diffraction grating or a prism, it does not produce a continuous spectrum. Instead, a series of separate bright lines of particular colours is seen on a dark background. This is a line emission spectrum.Each line is produced by electrons de-exciting between two particular energy levels. Because the energy levels are discrete, only certain photon energies, and so certain wavelengths, can be emitted:λ = hc / (E₁ − E₂)When white light (a continuous spectrum) passes through a cooler gas, the atoms absorb only the photons whose energies exactly match a gap between energy levels. This gives a line absorption spectrum: a continuous spectrum crossed by dark lines.For a given element, the dark absorption lines are at exactly the same wavelengths as the bright emission lines, because the same energy gaps are involved. The absorbed energy is re-emitted in all directions, so less light reaches the observer at those wavelengths.Line spectra are important evidence that electrons in atoms occupy discrete energy levels. If electrons could have any energy, atoms would emit a continuous spectrum.Every element has a unique set of energy levels, so each element has a unique line spectrum, like a fingerprint. Astronomers identify the elements in stars from the dark absorption lines in starlight. Helium was discovered in this way, from lines in the spectrum of the Sun, before it was found on Earth.An atom excited to the n = 3 level can emit three different lines: 3 → 2, 2 → 1 and 3 → 1.