Emission and absorption spectra

Three main types of spectrum

When light is dispersed using a prism or a diffraction grating, one of these three types of spectrum is observed:

Type of spectrum	Typical source	Appearance
Continuous	Heated solid, liquid or dense gas (filament, lava)	All colours follow one another without interruption
Emission lines	Hot, low-pressure gas (sodium vapour lamp, neon tube)	A few isolated coloured lines against a black background
Absorption spectrum	White light passing through a cooler gas	A continuous spectrum crossed by black lines

Where do the lines come from?

Each atom or ion can only absorb or emit light at very specific wavelengths, which depend on its electronic structure. An excited atom returns to a lower energy level by emitting a photon of a characteristic wavelength: this produces an emission line. Conversely, if white light passes through a cooler gas, the atoms in that gas absorb precisely these same wavelengths, creating dark (absorption) lines at the same position in the spectrum.

A unique signature

The wavelengths of the lines (whether emission or absorption) form a sort of identity card for each chemical element: two different elements (for example, hydrogen and sodium) never have exactly the same lines.

A common misconception

Do not say that the continuous spectrum has no lines simply because there is no gas: rather, in a solid or a gas under high pressure, the energy levels of the atoms are so disturbed by particle interactions that all wavelengths become possible, hence the absence of isolated lines.