A spectrometer is an instrument that measures properties of light. Spectrometers often use a device called a diffraction grating, which consists of many finely spaced slits or grooves. When light passes through or reflects and diffracts off the grating, it is separated into its component wavelengths, creating a spectrum. By studying the resulting spectrum and looking at the intensities of the different wavelengths of light, it is possible to identify the chemical composition of matter.

Atoms and molecules absorb and emit light at particular wavelengths, producing characteristic patterns of electromagnetic radiation. For example, hydrogen has a specific set of wavelengths of light that it absorbs and emits, which can be used to identify it. A hydrogen atom contains one proton (in its nucleus) and one electron. Electrons can be thought of as occupying specific energy levels outside the nucleus. The electron in the hydrogen atom is usually at the lowest energy level, known as the ground state. However, the electron can jump to a higher energy level when the atom absorbs energy, such as when it is heated or absorbs a photon (a particle of light). The excited electron cannot stay at the higher energy level; it naturally wants to drop down to the ground state. When this happens, a photon of light is emitted by the atom.

There are specific energy levels that the electron can jump between, and each possible transition absorbs or emits a photon at a specific wavelength that corresponds to the energy difference between the levels. An absorption line is produced when the electron jumps to a higher level; an emission line is produced when the electron drops to a lower level. Because only certain energy levels are possible, only photons of specific energies can be absorbed or emitted by the atom. This results in characteristic spectral lines at particular wavelengths.

Each chemical element has its own unique patterns of spectral lines that correspond to its set of allowed energy levels. Similarly, molecules also have characteristic spectral lines, based on the elements that compose them. Thus, these patterns of absorption or emission lines can be used to identify the chemical composition of objects.

The information provided by line spectra can be useful in many applications. Astronomers use spectrometry to study the light from galaxies, stars, and planets. There are also practical applications for spectrometry on Earth, such as pharmaceutical chemistry, where it can be used to determine the quantity of a chemical in a drug.

Spectral lines can be formed not only by visible light, but across the entire electromagnetic spectrum. However, because the spacing of the lines of a diffraction grating are related to the wavelength of light that it defracts, gratings become impractical at very large wavelengths such as radio waves. In order to study spectra at the radio end of the electromagnetic spectrum, scientists use other types of specialized instruments.

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