Why do photosynthesizers contain accessory pigments in addition to chlorophyll a?

A pigment is any substance that absorbs light. The color of a pigment is the result of the reflected (unabsorbed) wavelength. Chlorophyll, the green pigment of all photosynthetic cells, absorbs all wavelengths of visible light except green, which is reflected and perceived by our eyes. A blackbody absorbs all the wavelengths it receives. The white pigment or light colors reflect all or almost all wavelengths. The colored substances have their characteristic absorption spectrum, which is the absorption pattern of a given pigment.

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