An absorption spectrum is a graphical representation that shows how a substance absorbs light at different wavelengths. When light passes through a gas or liquid, certain wavelengths are absorbed by the atoms or molecules, resulting in dark lines or bands on the spectrum. These lines correspond to specific energies that match the energy levels of electrons in the atoms. Scientists use absorption spectra to identify the composition of materials, such as stars or gases. Each element, like hydrogen or helium, has a unique absorption spectrum, allowing researchers to determine what elements are present in distant celestial bodies or laboratory samples.