What causes an absorption line in a spectrum?

An absorption line in a spectrum occurs when light passes through a cooler gas or medium, and certain wavelengths of light are absorbed by specific atoms or molecules within that gas. This process happens because the atoms or molecules have energy levels that correspond to particular wavelengths of electromagnetic radiation. When light containing a range of wavelengths, such as the light from a star, passes through this gas, the specific wavelengths that correspond to the energy difference between electron energy levels in the atoms are absorbed, leading to the formation of dark lines in the spectrum at those wavelengths. These absorption lines provide valuable information about the composition, temperature, density, and motion of the gas through which the light has passed.

The other options describe processes that do not lead to the formation of absorption lines. Emission of light creates bright lines in a spectrum, whereas scattering and reflection involve light altering direction but do not remove light at specific wavelengths from the spectrum as absorption does. Thus, the absorption of light at those specific wavelengths is the definitive cause of an absorption line.