Fraunhofer lines

The German physicist Joseph von Fraunhofer (1787–1826) invented a device called a spectroscope which contained a diffraction grating. When he used this device to analyse the light coming from the Sun he discovered dark lines on a continuous spectrum. These are called Fraunhofer lines.

Diagram showing Fraunhofer lines - dark lines on a continuous spectrum.

We now know that these dark lines are absorption lines. But how are they produced, and what do they show us?

The Sun and other stars produce all wavelengths of light. As this light passes through the cooler outer atmosphere, gas atoms absorb certain wavelengths of the light, producing a line absorption spectrum which we see from Earth.

Scientists in the 19th century were able to compare these dark lines with the line emission spectra of the known elements and identify what elements were in the cooler atmosphere.

Can you see sodium (Na), iron (Fe), calcium (Ca) and magnesium (Mg)?

The line emission spectra of a range of common elements.

When the light from a distant gas cloud (nebula) is examined, it shows a line absorption spectrum. The line emission spectra of hydrogen (H), mercury (Hg) and neon (Ne) are shown. Which of these elements is in the gas cloud?

Above are the the emission spectra of hydrogen, mercury and neon. Below is the absorption spectrum from a gas cloud.

The line emission lines and the black absorption lines line up with hydrogen.