Kirchhoff's laws

Kirchhoff's laws (Kirchhoff's three laws of spectroscopy) indicate the source of spectral lines.

1. A hot solid, liquid, or dense gas produces a continuous spectrum.
2. A hot thin (i.e., low density) gas produces emission lines.
3. A continuous spectrum passing through a cold thin gas produces absorption lines.

Note that other widely-used physical laws are attributed to physicist Gustav Kirchhoff (such as Kirchhoff's law of thermal radiation) and another grouping is known as Kirchhoff's laws of circuits.