When white wave passes by a semi-transparent solid, or liquid or gas, it's spectrum have certain dark lines or bands, such spectrum is known as absorption spectrum (of the substance by which light is passed).
(i) Substances in atomic state produce line absorption spectra. Polyatomic substances such as H2, CO2 and KMnO4 produce band absorption spectrum.
(ii) Absorption spectra of sodium vapor have two (yellow lines) wavelengths D1 (5890Å) and D2(5896 Å)