Absorption Of Radiation
An absorption spectrum is a measure of the wavelengths of radiation that a pigment absorbs. The selective absorption of different wavelengths determines the color of a pigment. For example, the chlorophylls of higher plants absorb red and blue wavelengths, but not green wavelengths, and this gives leaves their characteristic green color.
The molecular structure of a pigment determines its absorption spectrum. When a pigment absorbs radiation, it is excited to a higher energy state. A pigment molecule absorbs some wavelengths and not others simply because its molecular structure restricts the energy states which it can enter.
Once a pigment has absorbed radiation and is excited to a higher energy state, the energy in the pigment has three possible fates: (a) it can be emitted as heat, (b) it can be emitted as radiation of lower energy (longer wavelength), or (c) it can engage in photochemical work, i.e., produce chemical changes. Flavonoids, carotenoids, and betalains are plant pigments which typically emit most of their absorbed light energy as heat. In contrast, chlorophyll, phytochrome, rhodopsin, and phycobilin are plant pigments which use much of their absorbed light energy to produce chemical changes within the plant.