An absorption spectrum represents the absorption of a molecule as a function of wavelength.
Y-axis = absorbance A or transmittance T
Aλ = log I0/I without unit
Tλ = 10-Aλ = I/I0
On the X-axis = wavelength in nm
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Chemical groups responsible for absorption are called chromophores.
For gases, absorption bands are thin, although for liquids they can be tens of nanometers wide.