Each atom has a characteristic spectrum , which can be considered like a fingerprint . The spectra allow us to identify the elements. Not only do atoms give off energy, but they absorb it too. So if white light is passing through some material, then certain wavelengths will be absorbed, and will be missing in the spectra. These are called absorption lines .
Astronomers use emission lines to identify the composition of stars.
They use absorption lines to identify the composition of interstellar dust and atmospheres of the planets. The branch of science that identifies things this way is called spectroscopy . The word laser is an acronym for light amplification by stimulated emission of radiation Most lasers are made of some material such as a crystal of ruby or a gas that is enclosed in a glass tube, like argon. Suppose an atom was in an excited state. I.e. an electron was in a higher energy orbit.