Summary

A prism (or an array) is used to break a beam of light according to its different frequencies. The spectrum obtained can be continuous or discrete ("line spectrum").

One of the great discoveries of quantum mechanics is that the energy of an atom can only have certain well-defined values. It is "quantized" (see animation line spectrum of the hydrogen atom). For this reason, a gas composed of a single atom can absorb or emit a limited number of frequencies.

For a given element, the emission spectrum (upper part of the animation) has the same frequency as its absorption spectrum (bottom part).

source for the values of spectral lines: CDS Strasbourg University (link) from Reader J., and Corliss Ch.H. CRC Handbook of Chemistry and Physics; NSRDS-NBS 68 (1980).

Click on an element to select it.

Learning Goals

• To distinguish between absorption spectra and emission spectra.
• To characterize an atom by its light spectrum and to introduce spectroscopy.
• To understand that the spectral emission lines of an element have the same frequencies as the lines of the absorption spectrum.

Learn More

If the atom receives a "quantum" of energy in the form of a photon, for example, it can absorb this energy. Electrons gain energy and enter an excited state. This is an atomic transition. Spontaneously and randomly, the electron tends to go down to its ground state by "jumping" from a high energy state to a lower energy state. It then emits a quantum of energy as a photon (light energy). This photon is sent in a direction that is unlikely to be the same as the absorbed photon, which results in a dark stripe on the absorption spectrum.

The line spectrum (light) emitted by an atom is a very precise testimony of its electron configuration. It is this "atomic signature" that allows us to know the precise chemical composition of a star or an interstellar cloud simply by spectral analysis of the light emitted or absorbed.