Types of spectra

We can classify the spectra in two ways.

By distribution:

• Continuous spectrum: contains all wavelengths in a continuous, unbroken way (a rainbow band), e.g. from a glowing solid like a filament or the Sun's photosphere. It depends on temperature, not on the material.
• Line spectrum: only certain separated wavelengths (bright lines on a dark background), as from excited atoms of a gas. The lines are characteristic of the element, so line spectra serve as "fingerprints" of the elements.

By origin:

• Emission spectrum: produced when excited atoms fall from higher to lower levels and emit photons.
• Absorption spectrum: when white (continuous) light passes through a cool gas, the atoms of the gas absorb photons of exactly those energies that correspond to their allowed jumps. The transmitted light has dark lines at positions where wavelengths are missing. These missing wavelengths are exactly the wavelengths that the same gas emits in its emission spectrum.
  • Fraunhofer lines in the solar spectrum are an absorption spectrum: the cooler gases in the Sun's atmosphere (mainly hydrogen and helium, among other elements) absorb these wavelengths from the Sun's continuous radiation.
  1Source

  Emission line spectrum

  • hot (excited) gas

  Absorption spectrum

  • cool gas in front of a continuous source

  2Appearance

  Emission line spectrum

  • bright lines on dark background

  Absorption spectrum

  • dark lines on a bright continuous background

  3Cause

  Emission line spectrum

  • electrons fall to lower levels

  Absorption spectrum

  • electrons rise to higher levels

  4Positions

  Emission line spectrum

  • same wavelengths for the same element