Living things require energy to stay alive. The main energy source for autotrophs, a group of organisms that produce their own food, achieve this by gathering natural commodities such as water and sunlight. These sources of energy are converted through a series of biochemical l processes into substances that the autotroph can use to maintain its necessary functions. The sun is the starting point for the process of photosynthesis ("photo" refers to light sunlight). The sun emits many, many different wavelengths, including X-rays, UV rays, and a spectrum of light that is visible to us. The light used by photosynthesis is the spectra of visible light.
The figure above shows the spectra of light which is absorbed by a plant. The sun is sometimes thought to emit simply a white light, but sunlight is actually reflected in a spectra similar to the one shown above. Plants appear to have a green color because this green light is not absorbed as much as other colors (with different wavelengths). As a result, this light is reflected by the plant, showing its green color.
Another feature of the above graph is the inclusion of several pigments in the typical autotroph. Organisms that undergo photosynthesis contain a variety of colored pigments, which are organized on membranes within chloroplasts. These pigments include Chlorophyll a, Chlorophyll b, and various carotenoids. Cholorophyll a is considered a primary pigments, while chlorophyll b and other light-absorbing carotenoids are referred to as accessory pigments. These accessory pigments serve mainly to broaden the spectra of absorbable light.