Where do the lines on an absorption spectra come from?

The lines on an absorption spectrum result from the absorption of specific wavelengths of light by atoms or molecules as they transition from a lower energy state to a higher energy state. Each element has a unique set of energy levels, so the absorbed wavelengths correspond to these specific energy differences. When light passes through a sample, certain wavelengths are absorbed, creating dark lines in the spectrum at those specific frequencies. The pattern of these lines serves as a "fingerprint" for identifying the composition of the material.