Spectral line
In order to understand spectral lines, color has to be understood. In a rainbow, the colors of light go from purple to red. Several hundred years ago, the famous scientist Isaac Newton did an experiment where he showed that even white light from the Sun was made up of all the colors of the rainbow. When other scientists following him looked at this rainbow very closely, they noticed that there were dark lines breaking up the rainbow, where certain shades of colors should be. These lines were very small, but could be seen when the rainbow got stretched out very far. They investigated this further, and discovered that certain chemical elements, like the hydrogen and helium that make up the Sun, absorbed certain frequencies of light where those colors should be, like a sponge absorbs water, but does not absorb chalk.