The primary source of energy that drives the process of photosynthesis is light. Light can either be reflected or absorbed, and in the case of autotrophs, roughly 45% of it is absorbed. Autotrophs use light-capturing pigments to absorb the energy needed to drive the conversion of carbon dioxide into glucose and oxygen. Light travels in waves having different lengths, and different pigments absorb light of each of those wavelengths. If white light is shone onto a pigment, the color that is seen indicates the wavelength of light that is being reflected. The other wavelengths are being absorbed. As such, chlorophyll reflects green light and absorbs the other colors of the visible spectrum.
The quantity of light that is absorbed by pigments provides evidence as to how well the wavelength works to energize the molecules during photosynthesis. There are three main pigments used in photosynthesis, chlorophyll a, chlorophyll b, and carotenoids. The green, light-capturing pigment used directly in photosynthesis is called chlorophyll a. It works directly during the light reactions. Chlorophyll b, which is also green, is an accessory pigment that assists with the capture of energy, transferring it to chlorophyll a. Carotenoids may be red, orange, yellow, or brown. They cannot transfer captured light energy directly to the reactions that make up photosynthesis. Instead, these pigments must transfer their energy to chlorophyll. Another function of carotenoids is to protect the cells from excess light energy. They absorb and disperse excessive amounts of light that may hit the leaves and otherwise damage chlorophyll molecules. Phycobilins, another type of photosynthetic pigment, are not found in plants but are found in algae and cyanobacteria. These pigments may be red or blue. Unlike carotenoids, phycobilins may be directly involved in photosynthesis.
To graphically represent the effectiveness of light for photosynthesis, an action spectrum or an absorption spectrum can be created. An action spectrum is a graph that shows how the rate of photosynthesis is affected by different wavelengths of light. An absorption spectrum is a graph that shows how different pigments absorb light. In both cases, photosynthesis works best when blue, violet, and red wavelengths are present. Since green wavelengths are reflected, they are the least useful.