• Tags: emission, absorption, spectra
• Difficulty: Foundation

Emission spectrum: produced when excited atoms transition to lower energy states. Shows bright lines on a dark background. Contains all possible transitions from the excited states present. Absorption spectrum: produced when white light passes through a cold gas. Shows dark lines on a continuous background. Only transitions from the ground state (or populated levels) are seen — the gas absorbs specific wavelengths and re-emits in all directions.

For hydrogen at room temperature, almost all atoms are in the ground state (n=1). The absorption spectrum shows only Lyman series lines (transitions from n=1 upward). Balmer series absorption is not observed at room temperature because very few atoms are in n=2. At very high temperatures, n=2 becomes populated and Balmer absorption appears.