the Fraunhofer spectrum

Joseph von Fraunhofer

Fraunhofer not only confirmed Wolston's results, but also found that there were far more dark lines in the spectrum than Wolston had suspected. Fraunhofer showed that these were a feature of sunlight and not an illusion nor an optical effect, and he labelled them with letters of the alphabet (A,B,C etc.). We now call these dark lines Fraunhofer lines.

What are the Fraunhofer lines and how are they formed?
When the visible light from below the Sun's surface passes through the layers above it (the photosphere and chromosphere), some of the light at particular wavelengths is absorbed by atoms and ions and so is missing in the spectrum we see. When there is no light it appears as black in the spectrum.

Here is a spectrum showing Fraunhofer lines

The dark lines in the spectrum occur when light is absorbed at a particular wavelength.

K and H are two lines from calcium at 396.8 and 393.4 nm

D corresponds to two lines from sodium at 589.0 and 589.6 nm

C is a line from hydrogen at 656.3 nm

When light passes through material and is absorbed by atoms and ions of each element (hydrogen, calcium, sodium, iron etc.) a unique set of dark lines is formed in the spectrum. These are called absorption lines.