Light is a form of energy that can act as both waves and subatomic particles called photons. Each wavelength of light corresponds with photons of a specific energy level. Long wavelengths such as red light have low-energy photons, and short wavelengths such as violet light have high-energy photons.
Pigments are light-sensitive molecules (such as chlorophyll) that absorb certain wavelengths of light and reflect others. When a pigment molecule absorbs light, it changes from its ground state to an excited state. In other words, the photons are striking the molecule with the exact amount of energy needed to move electrons from inner, lower-energy orbitals to outer, higher-energy orbitals. Photons that do not have the energy to excite the molecule are reflected, not absorbed. It is this reflected light that we see.
There are several types of chlorophyll. The main types found in plants are chlorophyll a and chlorophyll b. These pigments absorb blue and red light for photosynthesis and reflect green light that we can see.
Plants also contain other pigments such as carotenoids, which are used for various purposes. During the spring and summer, plants appear green because they contain far more chlorophyll than other pigments. As daylight decreases in the fall, plants stop producing chlorophyll and begin to seal off their leaves from the branches in preparation for winter. The chlorophyll soon breaks down, allowing us to see the colors from carotenoids and anthocyanins before the leaves are completely sealed off from the plant and fall to earth.
