What physical phenomenon is responsible for the appearance of dark lines in an absorption spectrum?

The appearance of dark lines in an absorption spectrum is primarily due to the absorption of light. When light passes through a gaseous medium, certain wavelengths of light are absorbed by the atoms or molecules within that medium. This absorption occurs when electrons in the atoms or molecules move from a lower energy state to a higher energy state, corresponding to specific energies of photons, or light particles.

The result of this process is that those particular wavelengths of light do not reach the observer, creating dark lines at those specific wavelengths in the spectrum. Each element has a unique pattern of absorption lines, which allows scientists to identify the presence of specific elements in distant stars and gases.

This mechanism is distinct from reflection, where light bounces off surfaces without being absorbed, or emission, where light is produced by excited atoms releasing energy. Scattering, on the other hand, involves light being redirected, but not removed from the spectrum completely. In the case of absorption spectra, it is the direct absorption of specific wavelengths that leads to the formation of the dark lines characteristic of this phenomenon.