Image: A diagram of a simple spectrometer.
Why do some light sources have different line spectrum? Of different colors? This is due to the emission spectrum. Atoms of different elements have different electronic structures, thus, light emitted and absorbed are would vary for every element when excited. The emission spectrum of an element is the pattern of light emitted with its own characteristic pattern of frequencies. Therefore, every element had its own specific line spectrum.
Image: Line Spectrum of atoms
When element is heated to incandescence, it emits light that is unique to the atomic structure of the element. The light emitted is split through the spectrometer, due to the difference in frequency of light waves, thus, producing a unique line spectra of the element. Specific light frequencies give rise to sharply defined bands on the scale.
The line spectra of light emitted of elements is known as emission spectra. However, elements can absorb light too, and the absorption spectra of an element can be identified using a spectrometer. Thus, the spectrometer is able to identify both the emission and absorption spectra of elements.
Examples:
Image: Line spectra of sodium. Sodium has a very characteristic yellow band in its emission spectra and can be seen when tested with sodium vapor lamp on a spectrometer.
Image: Emission spectrum of excited (heat hydrogen gas) consists of distinct spectral lines.